Image SEO: how to optimise every image on your website for search

Why image optimisation matters for SEO

Images affect search performance in two separate ways, and most site owners address neither consistently. The first is speed: images are typically the largest files on any web page, and uncompressed images are the single most common cause of slow page load times. Page speed is a confirmed Google ranking factor, and the gap between a fast page and a slow one is measurable in both rankings and visitor behaviour. The second is discoverability: Google indexes images independently, and a well-optimised image can appear in Google Image Search, bringing visitors to your site from searches that your text content would never capture.

The good news is that image SEO requires no specialist technical knowledge. The changes are applied before and during the upload process, not after. File naming, format conversion, compression, and writing alt text are all tasks that take a few minutes per image and compound in value across an entire site.

Image optimisation also intersects with accessibility. Alt text, one of the two most important image SEO practices, simultaneously helps Google understand what an image shows and helps visually impaired users understand the same thing via screen readers. Optimising images for SEO and optimising for accessibility are not separate tasks; the same actions serve both goals.

The on-page SEO guide covers how image optimisation sits within the broader framework of page signals, alongside title tags, content depth, and internal linking. Images are one of the faster on-page fixes available because the improvements do not require changing the written content of a page.

Image file naming for SEO

Every image file has a name. By default, cameras and design tools assign meaningless names: IMG_4923.jpg, screenshot-2026-01-14.png, Untitled-1.webp. These names tell Google nothing about what the image contains. Descriptive file names tell Google what the image shows before it even processes the image itself.

The rule is straightforward: name the file after what it depicts, using your target keyword where it naturally describes the image. A screenshot of a Rank Math settings panel used in an article about on-page SEO should be named rank-math-on-page-seo-settings.webp, not screenshot-1.webp. A product photograph of a blue running shoe should be named blue-running-shoe-side-view.webp, not product-image-4.webp.

Use hyphens between words in file names, not underscores or spaces. Google treats hyphens as word separators. Underscores are treated as joining characters, which means blue_shoe.webp is read as one word rather than two. Spaces in file names are converted to %20 in URLs, which looks unprofessional and can cause technical issues in some systems.

Keep file names concise. Three to five words is a good target. A file name that lists every keyword variant you are targeting is not a file name; it is keyword stuffing, and Google's systems apply the same quality signals to file names that they apply to anchor text and meta tags.

Rename images before uploading them, not after. Renaming an image that is already live on a page changes its URL, which breaks any cached version and can cause a brief crawl error until Google re-indexes the new URL. Build the renaming step into your publishing workflow before images are ever uploaded.

How to write alt text that helps both users and Google

Alt text is the attribute added to an image's HTML tag that describes what the image shows. It appears in place of an image if the image fails to load, is read aloud by screen readers to visually impaired users, and is used by Google to understand the content of an image since Google cannot interpret images as reliably as a human reader can.

Write alt text that describes what the image actually shows. This sounds obvious, but the two most common errors are writing alt text that describes what the image is for rather than what it shows ("SEO guide image" instead of "screenshot of Google Search Console showing a click-through rate drop in April") and forcing the target keyword into alt text for images where it does not naturally apply.

The target keyword should appear in alt text only where the image genuinely depicts something related to that keyword. For a page about image SEO, an image showing a compressed WebP file being uploaded to a CMS could legitimately include the phrase "image SEO" in its alt text. A decorative background image on the same page should use empty alt text (alt=""), not a forced keyword insertion.

Keep alt text under 125 characters. Screen readers typically stop reading alt text after this length. Longer descriptions add no SEO value and create a poor experience for assistive technology users. If an image genuinely requires a longer explanation, use a caption below the image rather than loading the alt attribute with a paragraph of text.

For e-commerce product images, alt text should describe the specific product variant shown: colour, model, orientation, and any distinguishing features visible in the image. For infographics and data visualisations, the alt text should summarise the key information the image communicates, since the data within the image is not readable by Google unless it is also present as text on the page.

Image compression and Core Web Vitals

Core Web Vitals are Google's set of page experience signals that contribute to ranking. The three metrics, Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS), each reflect a different aspect of how fast and stable a page feels to a visitor. Images affect LCP most directly: LCP measures how long it takes for the largest visible content element to load, and on most pages, the largest visible element is an image.

Compressing images before uploading them is the single most effective way to improve LCP. An uncompressed JPEG photograph from a standard camera can weigh 4 to 8 megabytes. The same image compressed for web use should weigh between 50 and 200 kilobytes without visible quality loss. That is a reduction of 95 percent or more in file size, which directly reduces the time taken for the page to load.

WebP is the recommended format for virtually all web images. Developed by Google, WebP delivers file sizes roughly 25 to 35 percent smaller than JPEG at the same visual quality, and supports both lossy and lossless compression as well as transparency. Most modern browsers support WebP, and all major website platforms accept it on upload. Converting existing image libraries from JPEG and PNG to WebP is a one-time technical task with lasting speed benefits.

Tools like Canva and Adobe Express export images in WebP format with compression controls. Adobe Stock and Pexels provide stock images that still need compression before use on your site. For AI-generated images created with tools like Midjourney or Krita, always export in WebP and run compression before uploading.

Cumulative Layout Shift is affected by images that do not have defined width and height attributes in their HTML. When a browser loads a page before the images are fully loaded, it cannot reserve the correct space for them. Images then pop into place as they load, shifting the content around them and creating a jarring experience. Adding explicit dimensions to image HTML tags prevents this. Most modern CMS platforms handle this automatically.

Creating images that earn links and shares

Beyond technical optimisation, the content of an image can be an active SEO asset. Images that contain original data, unique visualisations, or genuinely useful diagrams attract inbound links from other sites that want to reference your image in their own content. Inbound links from credible sources are a significant ranking signal. An infographic that summarises a complex topic better than any competing resource can generate links passively over time, contributing to the domain authority that supports all your other rankings.

Original images also differentiate your content from competitors who rely on the same stock photography. A page that includes screenshots of real tool interfaces, custom diagrams, or original photographs signals to both readers and Google that the content is produced by someone with genuine hands-on knowledge of the subject. This feeds directly into E-E-A-T signals, particularly the Experience and Expertise dimensions that Google's quality raters assess.

For data-driven content, original charts and graphs created from your own research or analysis are highly linkable. Embed the underlying data as accessible text on the page wherever possible, since Google cannot read data inside an image. The image communicates the finding visually; the text communicates it to Google's crawlers.

For how-to content, annotated screenshots are more valuable than stock photography. A guide explaining how to set up a Rank Math sitemap is more useful with screenshots showing each step in the interface than with a generic image of a laptop. The screenshots also help differentiate your content from the dozens of other articles covering the same topic with identical stock imagery.

The full picture for how image optimisation fits within your on-page strategy is covered in the on-page SEO factors guide, which includes how images interact with page speed, content depth, and the checklist of elements to review before publishing any page. Tools like Semrush and Rank Math both flag missing alt text and oversized images in their site audit features, making it straightforward to identify image SEO issues across an existing site. Platforms like Webflow, WIX, and Squarespace all include image optimisation and lazy loading as standard, reducing the manual steps required for sites built on those platforms. Google Analytics paired with Search Console allows you to identify which pages have the slowest load times and correlate those with ranking positions, giving you data to prioritise your image audit work.

What this means for your visual SEO

Image SEO is one of the most consistently neglected areas of on-page optimisation, which makes it one of the faster wins available on sites that have been publishing images without a systematic approach. The changes are not complex. Rename files before uploading, write descriptive alt text for every informational image, convert to WebP, and compress before upload. Applied consistently across new content and as a one-time audit on existing pages, these steps reduce page load times, support rankings, and improve accessibility simultaneously.

For sites with large image libraries, prioritise the pages that rank in positions six through twenty for their target keywords. These pages are close enough to rank that a page speed improvement or better alt text could tip them into the top five. Use Google Analytics and Search Console together: Search Console shows ranking positions, and the Core Web Vitals report in Search Console identifies which specific pages have poor LCP scores tied to images.

For new content, build image optimisation into the publishing checklist rather than treating it as a separate pass after the page is live. Rename, compress, and write alt text at the time of upload. A two-minute habit per image compounds across every page you publish into a meaningfully faster, better-ranked site.