How to Manually Optimise Images for the Web

JavaScript frameworks and build tools like Next.js, Astro, and Vite perform optimisations on images to improve the performance of websites. If you’re not using any tools that does this automatically, here are some things you can do to optimise the images in your website.

1. Convert the images to .webp or .avif formats. These are the best formats for images on the web, as they are smaller in size but still have a similar image quality to larger formats like .png. In terms of image size, .avif is smaller than .webp, but both formats would still work well. Here’s a command you can run to convert formats from the command line:^[1]

   # this converts the image format. you can also specify the output file name
   gm convert image.jpg image.webp
   
   # you can shrink the image size even further at the cost of image quality
   gm convert -quality 75 image.jpg image.webp

2. Set the width and height attributes. Just use the image’s size if you’re not resizing it. This is needed to prevent cumulative layout shifts (CLS).

3. Set loading="lazy". This will defer the loading of the image until it is almost scrolled into the viewport. There’s an exception to this. For important images, such as the largest contentful paint (LCP) image, set loading="eager". SvelteKit also recommends to set fetchpriority="high" for this.

4. Set decoding="async". This will defer the decoding of the image until after the other DOM content is loaded.

5. Set the alt attribute. Images aren’t always loaded. It’s just a good practice to have an alt text, except if the image is purely for decoration.

There are more optimisations you can do, such as using different image sizes depending on the user’s device size,^[2] but these should be enough for basic use cases.