Image Optimization for Core Web Vitals and SEO

Images and Core Web Vitals

Google's Core Web Vitals measure real-world user experience. Images affect two key metrics: Largest Contentful Paint (LCP), which measures loading speed, and Cumulative Layout Shift (CLS), which measures visual stability.

Optimizing for LCP

The LCP element is often a hero image or the largest visible image on the page. To improve LCP:

• Serve correctly sized images: Don't serve a 2400px image in a 600px container.
• Use modern formats: WebP is 25-35% smaller than JPEG at equivalent quality.
• Implement lazy loading: Use loading="lazy" for below-the-fold images, but never on the LCP image itself.
• Preload the LCP image: Use to start loading early.

Preventing CLS

Layout shifts caused by images happen when the browser doesn't know the image dimensions before it loads:

• Always include width and height attributes on tags.
• Use CSS aspect-ratio for responsive images.
• Avoid replacing placeholder images with differently-sized final images.

SEO Image Best Practices

Alt Text

Write descriptive alt text that accurately describes the image content. Good alt text helps search engines understand image context and improves accessibility.

File Names

Use descriptive, hyphenated file names instead of generic ones. blue-cotton-t-shirt.webp is far better than IMG_20240315_001.jpg.

Image Sitemaps

For important images, include them in your XML sitemap to ensure search engines discover and index them.