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image seo core web vitals lcp performance technical seo

Image SEO Checklist for Core Web Vitals

Practical guide to image seo checklist: formulas, workflow, implementation pitfalls, and a direct execution playbook with Image Compressor.

By Tools Hub Editorial Team·Published January 14, 2026·Time to read: 8 min

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Image SEO Checklist for Core Web Vitals

Images are the largest contributor to page weight on most websites, and the number one cause of poor LCP scores. This checklist covers every optimization that affects Core Web Vitals.

LCP (Largest Contentful Paint)

The hero image is typically the LCP element. Target: under 2.5 seconds.

•[ ] Hero image under 200KB after compression. Use WebP at quality 80 or AVIF at quality 65.

•[ ] Preload the hero image so the browser fetches it before parsing CSS:

<link rel="preload" as="image" href="/images/hero.webp" type="image/webp" />

•[ ] Serve from CDN. Edge-cache the image so it loads from the nearest PoP, not your origin server. Typical improvement: 100-300ms.

•[ ] No lazy-load on hero image. The loading="lazy" attribute on the LCP image delays it. Only lazy-load below-fold images.

•[ ] Avoid CSS background images for hero. They cannot be preloaded and are discovered late by the browser.

CLS (Cumulative Layout Shift)

Images without dimensions cause layout shift when they load. Target: under 0.1.

•[ ] Set width and height attributes on every <img> tag:

<img src="photo.webp" width="800" height="450" alt="..." />

•[ ] Use aspect-ratio CSS for responsive images that need to scale:

img { aspect-ratio: 16 / 9; width: 100%; height: auto; }

•[ ] No dynamically injected images above the fold without reserved space.

INP (Interaction to Next Paint)

•[ ] Lazy-load all below-fold images to reduce main thread work during initial load:

<img src="photo.webp" loading="lazy" width="800" height="450" alt="..." />

•[ ] Decode async for non-critical images to prevent decode from blocking interactions:

<img src="photo.webp" loading="lazy" decoding="async" alt="..." />

Alt Text

•[ ] Describe the image's function, not its appearance. "Dashboard showing monthly revenue chart" is better than "image of a chart".

•[ ] Include keywords naturally but do not stuff. One relevant keyword per alt text.

•[ ] Decorative images get an empty alt: alt="".

•[ ] Product images should include the product name and key feature: alt="Nike Air Max 90 in white, side view".

Format and Responsive Serving

•[ ] Use WebP as primary format with JPEG fallback via <picture> (see Image Compression Pipeline).

•[ ] Use srcset for responsive images:

<img
  srcset="photo-400.webp 400w, photo-800.webp 800w, photo-1600.webp 1600w"
  sizes="(max-width: 600px) 100vw, 800px"
  src="photo-800.webp"
  alt="Product detail"
/>

•[ ] Do not serve 1600px images to 400px viewports. The srcset attribute lets the browser pick the right size.