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Responsive Image Checker

Check if images on any webpage are the correct size for their display dimensions. Find oversized images wasting bandwidth and slowing your page.

The tool fetches each image to compare its real pixel size against its rendered display size in your HTML.

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Why Image Size Matters for Page Speed and SEO

Serving an image at 1,200×800px when the browser displays it at 300×200px wastes 94% of the pixels downloaded. The user gets the same visual result — but downloads four times more data than needed. On mobile connections, that translates directly to a slower page and a worse experience.

Google measures this. Largest Contentful Paint (LCP) and Total Blocking Time are both affected by oversized image downloads. Images that are too large for their rendered dimensions are one of the most common and easiest-to-fix causes of poor Core Web Vitals scores.

LCP and Page Speed

Oversized images are a direct cause of slow Largest Contentful Paint. The browser has to download more bytes before it can render the image. For hero images, this is often the single biggest drag on your LCP score. Resizing to the actual display dimensions cuts download time in proportion to the pixel reduction.

Cumulative Layout Shift (CLS)

Images without explicit width and height attributes cause layout shift as they load. The browser does not know how much space to reserve, so the page reflows when the image appears. Adding width and height to every img tag — or using an aspect-ratio CSS rule — eliminates this CLS penalty.

Responsive Images with srcset

The srcset attribute lets you provide multiple image resolutions. The browser picks the best size for the screen automatically. A 1x image for small screens, a 2x image for retina — same visual quality, significantly less data on smaller devices. This is the recommended solution for responsive image delivery.

Format Matters Too

Beyond dimensions, image format affects file size. WebP images are typically 25–35% smaller than JPEG at equivalent visual quality. AVIF can be 50% smaller. Use the <picture> element with source tags to serve modern formats to browsers that support them, with a JPEG fallback for older ones.

How to Fix Oversized Images

The fix depends on where images come from:

  • 01.For static sites: Export images at the exact pixel dimensions they will display. Use ImageMagick, Squoosh, or TinyPNG to resize before uploading.
  • 02.For WordPress: Regenerate thumbnails after changing image sizes in Settings → Media. Use a plugin like ShortPixel or Imagify to auto-resize on upload.
  • 03.For React / Next.js: Use Next.js <Image> component which handles srcset, lazy loading, and format conversion automatically.
  • 04.For any site: Add width and height attributes to every img tag matching the actual rendered size. This prevents CLS even before you fix the file size.

Frequently Asked Questions

An image is oversized when its actual pixel dimensions are significantly larger than the size it displays on screen. For example, a 1,200×900px image displayed in a 300×225px container downloads four times more data than necessary. The tool flags images where the intrinsic size is more than 25% larger than the rendered size.
The tool reads the width and height attributes from your HTML img tags to determine rendered size. It then fetches each image and reads its actual pixel dimensions using PHP's getimagesize function. If your images are sized via CSS alone (without HTML attributes), the tool will flag them as missing dimensions — add width and height attributes to your img tags to give the browser and this tool accurate data.
The srcset attribute lets you provide multiple versions of an image at different sizes. The browser picks the most appropriate one based on the screen size and pixel density. This means mobile users download a smaller file while desktop users get the full resolution. Adding srcset is the standard approach for responsive images and is rewarded by Google's PageSpeed Insights.
Yes, indirectly but measurably. Oversized images slow down page load time, which affects Largest Contentful Paint (LCP) — a Core Web Vitals metric Google uses as a ranking signal. Pages with slow LCP tend to rank lower. Fixing oversized images is one of the highest-impact performance improvements most websites can make.
The tool gives a score based on how many of your measurable images are correctly sized (within 25% of their rendered dimensions) or use srcset. A score of 80 or above is good. Below 60 means several images are significantly oversized and worth addressing before your next SEO audit.
The tool fetches every image on the page to measure its real dimensions. Pages with many images, or images hosted on slow servers, take longer to scan. The timeout per image is 5 seconds. Images that cannot be fetched within that window are flagged as unmeasured rather than causing the whole scan to fail.

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