Images make up a large share of the data downloaded when you visit a typical web page. Optimising them is one of the most effective ways to make a website load faster, and it often requires no changes to the design at all.
Lossy and lossless compression
Lossless compression reduces file size without changing a single pixel, by storing the data more efficiently. Lossy compression goes further, discarding detail that the human eye is unlikely to notice. Done carefully, lossy compression can shrink an image dramatically while it still looks identical to most visitors.
Modern formats
Newer formats such as WebP and AVIF typically produce much smaller files than traditional JPEG and PNG at similar visual quality. Most modern browsers support them, and websites can serve them with a fallback for older browsers.
Serving the right size
A common mistake is uploading a photo straight from a camera, thousands of pixels wide, and displaying it in a small box on the page. Responsive images let the browser choose an appropriately sized version for each screen, so a phone does not download a file meant for a large desktop monitor.
Why it matters
Faster pages keep visitors engaged, reduce mobile data usage and are favoured by search engines. For an online shop or a blog, a lighter page can mean more readers and more sales.