Image SEO Audit Tool - Alt Text, Image Weight & Format Checker

Give it a URL. The page is opened in a real browser, every image it actually renders is collected — <img> tags and CSS background-image — and each one comes back with its weight, format, HTTP status, displayed size, natural size, alt text and the problems it has.

Output is JSON: one record per page, plus one summary record for the whole run.

What a real run returns

https://www.smashingmagazine.com, single page, default settings: 28 images, 12 issues — 9 images displayed far smaller than the file actually served, 3 images with no alt text. Formats found on that page: 10 SVG, 6 PNG, 12 undetermined.

What it checks

Issue code	Raised when
ALT_MISSING	The image has no alt attribute, or an empty one
HEAVY_WARN	The file is over 200 KB and up to 500 KB
HEAVY_CRITICAL	The file is over 500 KB
NON_NEXT_GEN	A JPEG or PNG heavier than 50 KB, where WebP or AVIF would be lighter
UPSCALED	The image is displayed larger than its own resolution, so it looks blurry
DOWNSCALED	The image is displayed at under 25 % of its resolution, so most of the downloaded bytes are wasted

Each family of checks can be switched off: checkAlt, checkWeight (weight and format), checkDimensions.

Input

Field	Type	Default	What it does
startUrl	string	—	Required. The page to analyze, or the page the crawl starts from
crawlUrls	boolean	false	Follow internal links and analyze several pages instead of one
maxPages	integer	10	Upper bound on the number of pages analyzed when crawlUrls is on
checkAlt	boolean	true	Report images with no alt text
checkWeight	boolean	true	Report heavy images and outdated formats
checkDimensions	boolean	true	Report images displayed at the wrong size
timeout	integer (ms)	15000	Time budget for loading a page, and for each image request
userAgent	string	Mozilla/5.0 (compatible; SEO-Image-Analyzer/1.0)	User agent sent to the site

Only links on the same domain are followed, and anchors, mailto: and tel: links are ignored. There is no sitemap reading: the crawl only sees links present on the pages it visits.

Output

One record per page (type: "page"), with its images and its own counters:

{
  "type": "page",
  "pageUrl": "https://www.smashingmagazine.com",
  "title": "Smashing Magazine — For Web Designers And Developers",
  "httpStatus": 200,
  "imagesCount": 28,
  "issuesCount": 12,
  "images": [
    {
      "imageUrl": "https://www.smashingmagazine.com/images/logo.svg",
      "alt": "Smashing Magazine",
      "displayed": { "w": 52.06, "h": 69.42 },
      "natural": { "w": 113, "h": 150 },
      "bytes": 2735,
      "bytesKB": 2.67,
      "format": "svg+xml",
      "mime": "image/svg+xml",
      "httpStatus": 206,
      "cacheControl": "public,max-age=31536000,immutable",
      "issues": ["DOWNSCALED"]
    }
  ],
  "summary": {
    "altMissing": 3,
    "heavyWarn": 0,
    "heavyCritical": 0,
    "nonNextGen": 0,
    "upscaled": 0,
    "downscaled": 9,
    "byFormat": { "svg+xml": 9, "png": 6, "unknown": 11, "svg": 1, "octet-stream": 1 }
  }
}

One summary record for the run (type: "site-summary"), always the last one:

{
  "type": "site-summary",
  "pagesCrawled": 1,
  "imagesTotal": 28,
  "issuesTotal": 12,
  "topIssues": [
    { "type": "DOWNSCALED", "count": 9 },
    { "type": "ALT_MISSING", "count": 3 }
  ],
  "byFormat": { "svg+xml": 9, "png": 6, "unknown": 11, "svg": 1, "octet-stream": 1 },
  "byProblem": { "ALT_MISSING": 3, "HEAVY_WARN": 0, "HEAVY_CRITICAL": 0, "NON_NEXT_GEN": 0, "UPSCALED": 0, "DOWNSCALED": 9 },
  "byStatus": { "200": 8, "206": 8 },
  "thresholds": { "HEAVY_WARN": 204800, "HEAVY_CRITICAL": 512000, "HEAVY_WARN_KB": 200, "HEAVY_CRITICAL_KB": 500 }
}

thresholds is there so the numbers behind the verdicts are never a black box.

What you are charged for

• Page Analyzed: one event per page record actually written to the dataset. A page that could not be loaded is reported with httpStatus: 0 and an empty image list, and is not charged.
• Image Analyzed: one event per image analyzed on those pages, after deduplication by URL.

The amounts are shown in the pricing section of this page.

Limits, stated up front

• Weight is read from HTTP headers, not by downloading the image. A server that returns neither Content-Length nor a byte range leaves bytes at null, and the weight and format checks are then skipped for that image. On the example run above this was the case for 12 of the 28 images.
• Inline images (data: URIs) are analyzed like the others, and their imageUrl field then holds the whole base64 string, which can be several thousand characters long.
• CSS background images have no alt text and no natural size, by construction. They are therefore always reported as ALT_MISSING, and never get an UPSCALED or DOWNSCALED verdict.
• The page is not scrolled. Images that a lazy-loading script only fetches once the visitor scrolls may be missing from the report, or reported without their final source.
• pagesCrawled counts every page record, including the pages that failed to load.

Typical uses

Pre-migration audit of a site's images, accessibility check on alt text, hunting down the heavy images that slow a page down, and checking that a WebP/AVIF migration actually reached every template.