What Is Image SEO? Definition and Basics

What is image SEO?

Image SEO is the work of making your images easy for search engines to find, understand, and show, without slowing your pages down. It covers what describes the image (alt text, file names, captions, surrounding text), how it is delivered (format, dimensions, compression, lazy loading), and how it is discovered (HTML <img> elements, image sitemaps, structured data).

Images can bring traffic directly from Google Images and image packs in regular results, and they affect the performance metrics of every page they appear on. The largest image on a page is often the Largest Contentful Paint element.

Updated September 30, 2026By the Hydrogen SEO team

How it works

The main levers:

Alt text. The primary description; see alt text.
File names. ceramic-pour-over-dripper.jpg says more than IMG_4471.jpg.
Context. Google reads captions, nearby text, and the page title. Put images near the text they illustrate.
Format. WebP and AVIF are usually much smaller than JPEG or PNG at similar quality. SVG suits logos and icons.
Dimensions. Serve images close to their display size, with srcset for different screens, and always set width and height to prevent layout shift.
Loading. Lazy-load images below the fold; never lazy-load the main hero image.
Discovery. Google indexes images in <img src>. CSS background images are not indexed as images.

Example

html

<img
  src="/images/ceramic-dripper-800.webp"
  srcset="/images/ceramic-dripper-400.webp 400w, /images/ceramic-dripper-800.webp 800w, /images/ceramic-dripper-1600.webp 1600w"
  sizes="(max-width: 600px) 100vw, 800px"
  width="800" height="600"
  alt="White ceramic pour-over dripper on a glass carafe"
  loading="lazy">

Descriptive file name, modern format, responsive sizes, explicit dimensions, real alt text, and lazy loading because it sits below the fold.

In WordPress

WordPress generates multiple sizes on upload, adds srcset and sizes, sets width and height, lazy-loads images by default, and in recent versions skips lazy loading on the first large content image and gives it fetchpriority="high". Recent versions also support WebP and AVIF uploads. Hydrogen SEO adds image entries to its XML sitemap, can generate alt text with AI or fill missing alt from a template, and uses featured images as fallback share images; see image alt text and the image alt text SEO post.

Common mistakes

Uploading 4,000-pixel camera originals for an 800-pixel slot.
Text baked into images with no text equivalent.
Lazy-loading the hero image, which delays LCP.
Missing width and height, causing layout shift as images load.

Common questions

What is the best image format for SEO?

There is no ranking preference between formats. WebP and AVIF usually give the smallest files, which helps speed; use SVG for logos and simple graphics.

Do I need an image sitemap?

Not usually. Google finds images in your pages' img tags. Image sitemap entries help when images are loaded in ways Google might miss, and many SEO plugins add them automatically.