Why does Lighthouse flag 'Properly size images' even when using WebP?

Lighthouse flags 'Properly size images' if the intrinsic pixel dimensions of your image are significantly larger than the CSS display box on mobile devices, even if compressed in WebP.

Which format is better for Google PageSpeed: AVIF or WebP?

AVIF delivers 20% to 35% higher compression efficiency than WebP for rich photographs. Modern best practice is to serve AVIF with a WebP fallback inside an HTML5 picture tag.

How to Fix "Serve Images in Next-Gen Formats" and "Properly Size Images" in Google Lighthouse

When running a Google PageSpeed Insights or Lighthouse audit, two of the most frequent performance warnings that drain 15 to 30 points from your mobile score are:

Serve images in next-gen formats: Triggered when legacy JPEG or PNG images can be served as WebP or AVIF for an estimated savings > 8KiB.
Properly size images: Triggered when the intrinsic image dimensions served over the network exceed the display size in the browser viewport by more than 25KiB.

The Core Web Vitals Penalty: Serving unoptimized images directly delays your Largest Contentful Paint (LCP). On simulated 4G mobile connections, serving a 3MB PNG hero image instead of a 120KB AVIF image adds 3.2 seconds of network stall, dropping your score straight into the red zone.

The Dual Codec Strategy: AVIF + WebP Fallback

While WebP is supported across 98% of browsers, AVIF (AV1 Image File Format) provides an additional 20% to 35% byte reduction for photographic content at identical perceptual quality. The optimal production architecture serves AVIF with a WebP fallback inside the semantic HTML5 <picture> element.

<picture>
  <!-- Preferred Next-Gen: AVIF -->
  <source type="image/avif" srcset="product-640.avif 640w, product-1280.avif 1280w" sizes="(max-width: 640px) 100vw, 1280px">
  <!-- Universal Modern Fallback: WebP -->
  <source type="image/webp" srcset="product-640.webp 640w, product-1280.webp 1280w" sizes="(max-width: 640px) 100vw, 1280px">
  <!-- Legacy Fallback with Explicit Dimensions to Eliminate Layout Shift (CLS) -->
  <img src="product-1280.jpg" alt="Optimized product asset" width="1280" height="720" loading="lazy" decoding="async">
</picture>
      

Eliminating "Properly Size Images" With Calibrated Presets

To eliminate the "Properly size images" audit, web developers should never serve desktop-scale 2560px or 1920px banners to mobile phones. PagePulse 100 automates this with four calibrated responsive web presets:

Hero Container (1280x720): Calibrated for desktop hero banners and wide carousels.
Retina Display (1024x576): Ideal for tablets and high-DPI cards.
Medium Responsive (640x360): The exact container size for mobile viewports, saving up to 80% bandwidth.
Square Grid (600x600): Standard e-commerce catalog thumbnails with crisp product detail.

The Desktop Solution: Batch Processing with PagePulse 100

Manually re-saving dozens of Photoshop files or relying on slow cloud conversion sites with upload queues is inefficient for modern development workflows. PagePulse 100 processes entire folders locally on your Windows PC with multithreaded performance:

Select your source folder containing raw PNGs, JPEGs, and videos.
Choose your target compression quality and container preset.
Click Process All to generate ready-to-deploy WebP, AVIF, and silent WebM video assets in seconds.

The Dual Codec Strategy: AVIF + WebP Fallback

Eliminating "Properly Size Images" With Calibrated Presets

The Desktop Solution: Batch Processing with PagePulse 100

Automate Next-Gen Image Optimization in 1 Click