Image SEO in 2026: File Names, Alt Text, Formats, and Core Web Vitals

Images are usually the largest thing on any web page, and they are the easiest SEO win most sites completely waste. Get image SEO right and you pick up rankings on Google Image Search, faster page loads, better Core Web Vitals, and stronger accessibility scores. Yes, image file names absolutely affect SEO, and so does almost everything else about how you handle images. Here is the no-nonsense guide to image optimisation in 2026.

I will cover the four things that actually matter: file names, alt text, format and compression, and how images impact Core Web Vitals. Skip the rest of the noise.

Do image file names affect SEO?

Yes. Google has confirmed multiple times that file names are a (small) ranking signal for both web search and image search. They are also one of the strongest signals Google has when an image has weak surrounding context. The classic example: IMG_4729.jpg tells Google nothing. merino-wool-hiking-socks-grey.jpg tells Google exactly what is in the image.

Three rules for file names:

Use hyphens, not underscores. Google reads blue-sneakers as two words but blue_sneakers as one.
Be descriptive but concise. Three to five words is the sweet spot.
Use lowercase only. Avoid spaces, special characters, and capitals to stop URL encoding issues.

Rename images before you upload them. Renaming after the fact in the CMS rarely changes the underlying file URL.

How to write alt text that actually helps SEO

Alt text exists for two reasons: accessibility (screen readers describe images to visually impaired users) and SEO (Google uses it to understand image content). Both matter. Both want the same thing: a clear, factual description of what is in the image.

Good alt text rules:

Describe what is actually in the image. If it is a photo of a black leather wallet, write “black leather bifold wallet”. Not “buy wallets online cheap”.
Include the target keyword if it fits naturally. Forced keyword stuffing is obvious and Google ignores it.
Keep it under 125 characters. Most screen readers truncate after that.
Skip phrases like “image of” or “photo of”. Screen readers already announce that it is an image.
Leave decorative images with empty alt (alt=""). Things like dividers, background patterns, or icons that already have visible labels.

Bad: alt="shoes nike runners best running shoes"
Good: alt="Nike Pegasus 41 running shoe in navy and white"

The right image format in 2026

The format wars are essentially over. Here is what to use:

WebP for almost everything. 25 to 35 percent smaller than JPEG at the same visual quality, supported by every modern browser.
AVIF for hero images and large photography where you want even better compression. Browser support is now strong enough to use it as a primary format with WebP fallback.
SVG for logos, icons, and any vector graphics. Tiny file size, infinite scaling, and indexable by Google.
JPEG only as a fallback for very old browsers. Most sites no longer need it.
PNG only when you need transparency and SVG will not work.

Avoid GIFs entirely in 2026. They are huge and outdated. Use a short MP4 or WebM video instead.

Compression: how small is small enough?

Aim for hero images under 200KB and inline content images under 100KB. Thumbnails should be under 30KB. If a single image on your page is over 500KB, you are bleeding speed.

Tools I use and recommend:

Squoosh by Google for one-off compression with full control
ImageOptim on Mac for batch processing
TinyPNG for quick browser-based compression
Built-in Next.js or WordPress image optimisation if your site supports it

Always resize before you compress. Uploading a 4000px wide photo and letting CSS scale it to 800px is one of the most common speed mistakes I see on audits.

How images affect Core Web Vitals

Core Web Vitals are still a Google ranking factor in 2026, and images are usually the biggest culprit when scores are bad. Three metrics, three image fixes:

Largest Contentful Paint (LCP)

Your hero image is almost always the LCP element. To improve it: serve it in a modern format, size it correctly for the viewport, preload it (<link rel="preload" as="image">), and never lazy-load it. Lazy-loading the LCP image is one of the most common mistakes on slow sites.

Cumulative Layout Shift (CLS)

Always set explicit width and height attributes on every image. This lets the browser reserve the space before the image loads, which prevents content from jumping around. Modern frameworks like Next.js handle this automatically. WordPress sometimes does not, depending on the theme.

Interaction to Next Paint (INP)

Heavy image processing in the main thread can hurt INP. Use lazy loading (loading="lazy") on all below-the-fold images so the browser does not waste resources decoding them on first load.

Image schema and structured data

For ecommerce product images, news article hero images, and recipe photos, add proper schema markup so Google can show enhanced results. Use ImageObject schema with contentUrl, caption, and creator properties where relevant. This is one of the things I check for in any website grader audit.

Responsive images: serve the right size to the right device

A 2000px hero image is wasteful on a phone. Use srcset and sizes attributes to let the browser pick the right size for the viewport. This is also why responsive web design in 2026 is non-negotiable: it is not just about the layout, it is about every asset on the page.

The quick image SEO checklist

Descriptive, hyphenated, lowercase file names
Real, accurate alt text (or empty alt for decorative images)
WebP or AVIF format
Under 200KB for hero images, under 100KB for inline
Width and height attributes set on every <img>
Lazy loading on below-the-fold images, never on the LCP
Responsive srcset for multiple viewport sizes
Image sitemap submitted to Google Search Console

If you want a proper audit of how your images are performing, including Core Web Vitals scores and missing alt text, run our free website grader. It will flag every image issue on your site in under a minute.

Or if you want me to look at it personally and tell you exactly what to fix first, whether the issue is image SEO, on-page SEO, or web design performance, book a free strategy call. No lock-in contracts, no juniors, straight talk only.