Image SEO audit: what Google (and your visitors) actually see
Search engines cannot see your images. They read the signals around them — the filename, the alt text, the format, the weight that slows the page down. Most image SEO guides hand you a checklist and wish you luck; this page turns that checklist into a live image SEO audit. Drop the images you plan to publish, type in the alt text you intend to use, and each file is scored out of 100 on the five factors that matter — with the fix written next to every flag.
It takes about thirty seconds per image, and it catches the mistakes that quietly cost rankings: the 4 MB photo straight from a phone, the IMG_1234.jpg filename, the missing alt text, the PNG photo that should have been a WebP. These are not exotic problems — they are the default state of images on most websites, which is exactly why fixing them is an easy win.
What the image SEO audit checks
1. File weight. Heavy images slow the page, and slow pages rank worse — Google's Core Web Vitals measure it directly. The audit passes anything under ~100 KB (right for content images), warns up to ~300 KB (acceptable for a hero), and fails anything heavier. These are practical targets from web-performance guides, not official Google rules — the principle is what counts.
2. Dimensions. An image under 600 px wide is flagged as too small for content use; 1200 px+ earns a pass, which also covers the 1200×630 px that social platforms want for link-preview (Open Graph) images. Small images get upscaled by browsers and look blurry — always publish the largest version you have.
3. Format. WebP and AVIF pass — they hold the same visual quality at roughly 25–35% smaller sizes than JPG. JPG gets a gentle warning (universal, but heavier); PNG is flagged when it is likely a photo rather than a graphic with transparency; GIF-for-stills and BMP/TIFF are called out as the mistakes they are.
4. Filename. IMG_1234.jpg tells search engines nothing; blue-running-shoes-side-view.jpg does. The audit fails camera-style and generic names, warns on capitals, spaces and underscores, and passes clean lowercase hyphenated names.
5. Alt text. You type the alt text you plan to use, and the audit scores it live: missing alt is a failure, text over ~125 characters a warning, "image of" openings a warning, repeated keywords a stuffing warning, and two-word vagueness a warning. Mark purely decorative images with the checkbox instead — empty alt is correct for those.
How to write alt text that ranks (and reads well)
Alt text has two jobs: it tells search engines what the image shows, and it tells screen-reader users what they are missing. Write it the way you would describe the image to a person on the phone — one focused sentence. "Golden retriever puppy playing in a sunny garden" beats both "dog" (too vague) and "dog dog dogs puppy puppies cute dog garden" (stuffing that helps nobody and can hurt).
The rules the audit enforces are the ones every major guide agrees on: keep it under ~125 characters, never start with "image of" or "picture of" (the screen reader already announces "image"), include your keyword only if it describes the image naturally, and leave alt empty for decorative images — a decorative divider does not need announcing. Context matters too: the same photo needs different alt text on a pet-adoption page versus a camera-review page, because the page topic changes what the image is for.
Image filenames Google can read
Filenames are one of the few ranking signals you fully control, and they cost nothing to get right. Before uploading, rename the file to describe the image: lowercase, words separated by hyphens, no dates or camera codes. IMG_20261008_143522.jpg becomes handmade-ceramic-mug-blue-glaze.jpg. The audit even suggests the filename for you — type your alt text and it generates the hyphenated name automatically, so the filename and the alt text agree with each other (which is exactly what search engines like to see).
This also pays off in Google Image Search, where the filename is a genuine ranking factor. Nobody searches for "IMG_1234" — but plenty of people search for exactly what your descriptive filename says.
The right format and size
Format and size are really one decision: the smallest file that still looks good. The modern answer is WebP (or AVIF where your audience's browsers support it) — same looks, a quarter to a third fewer bytes than JPG. JPG remains perfectly acceptable and opens everywhere; PNG is for graphics with transparency and logos, never for photographs; GIF is for nothing except short animations, and even there video or animated WebP is smaller.
For sizes, use these practical targets: content images under ~100 KB, hero images under ~300 KB, thumbnails under ~40 KB. If the audit flags your file as heavy, compress the image first — and if it flags the format, convert to WebP before you upload. Fixing both problems after publishing means re-uploading; the audit exists so you fix them before.
Lazy loading and the copy-paste snippet
Once the audit is green, you still have to write the HTML — and the HTML matters. The snippet this tool generates includes everything a modern image tag should have: a descriptive filename in src, your alt text, explicit width and height (so the page does not jump around while images load — that jump is Cumulative Layout Shift, a Core Web Vitals metric), loading="lazy" (offscreen images wait their turn, so the visible page loads faster), and decoding="async".
One honest caveat, shown right under the snippet: remove loading="lazy" for your hero image. The largest image on screen should load eagerly — lazy-loading it delays the very thing Google measures. Lazy loading is for everything below the fold.
Image SEO checklist — the 60-second version
- Compress every image under ~100 KB before uploading (heroes: ~300 KB).
- Rename files descriptively: lowercase, hyphens, no camera codes.
- Convert photos to WebP; keep PNG for graphics with transparency.
- Write alt text under ~125 characters, describing the image plainly.
- Size content images at least 600 px wide; share images 1200×630 px.
- Lazy-load below-the-fold images; never the hero.
- Add width and height attributes to prevent layout shift.
- Empty alt (
alt="") for decorative images.