Image SEO: Alt Text, File Names and Everything Else That Ranks

Compressing your images makes them fast. But fast is only half of image SEO — Google also needs to understand what each image shows before it can rank it in Google Images or count it toward your page's relevance. That understanding comes almost entirely from signals you control: file names, alt text, captions and markup. Here's the complete picture.

File names: the signal everyone wastes

Google reads file names. IMG_20260709_143502.jpg says nothing; blue-ceramic-coffee-mug.jpg says exactly what the image is. Rename before uploading:

• Use 3–6 descriptive words, separated by hyphens (not underscores).
• Lead with the subject, not the brand: handmade-leather-wallet-brown.jpg.
• Don't keyword-stuff — wallet-cheap-best-buy-wallet-leather-wallet.jpg reads as spam to both robots and humans.

Alt text: written for people, read by Google

Alt text exists for screen-reader users and for the moments an image fails to load. Google reads it because it's the most reliable description of image content. Write it accordingly:

• Describe what's actually in the image, specifically: "Golden retriever puppy sleeping on a striped blanket" beats "dog" and beats "cute puppy sleeping dog blanket photo".
• Keep it under ~125 characters — screen readers may truncate longer text.
• Skip "image of" / "picture of" — that's already implied.
• Decorative images (spacers, background flourishes) get an empty alt="" so assistive tech skips them.
• Include your target keyword only when it genuinely describes the image. One natural mention; never a keyword list.

Context around the image

Google weighs the text near an image heavily: the caption, the preceding paragraph, the page's heading structure. An image of a recipe step ranks for that recipe because it sits inside the recipe. Practical implications:

• Place images next to the text they illustrate, not in a gallery dump at the end.
• Use real captions (<figcaption>) where a caption helps the reader — captions are among the most-read text on any page.

Technical signals that complete the picture

• Dimensions in HTML: set width and height attributes so the browser reserves space — this eliminates layout shift, which is itself a ranking signal (CLS).
• Lazy loading: add loading="lazy" to below-the-fold images — but never to the main hero image, or you'll slow down your Largest Contentful Paint.
• Responsive images: use srcset to serve smaller files to phones. A 375 px-wide phone screen has no use for a 1920 px image.
• Structured data: product, recipe and article schema all have image fields; filling them makes your images eligible for rich results.
• Image sitemaps: for image-heavy sites (photography, e-commerce), listing images in your sitemap helps Google discover them.

And yes — compression is still the foundation

None of the above rescues a page that takes eight seconds to load. Google Images visibly favors pages that load fast, and image weight is the dominant factor. The full stack, in order: right format → right dimensions → compressed → descriptively named → alt-texted → placed in context. Each step takes seconds per image, and together they're the difference between images that cost you traffic and images that bring it.

Compress your images now

Ready to put this into practice? Use the free WebPcompress compressor — it runs entirely in your browser, handles 20 images at a time, and your photos never leave your device.