Serve images in next-gen formats
This warning requests you to use next-generation formats for your images. By “next-generation formats” Google refers to JPEG 2000, JPEG XR, and especially, WebP format.
Google Lighthouse will convert to WebP each BMP, JPEG, and PNG image on your page, if there are any potential savings after the conversion, you will see this recommendation.
You can read more here: https://web.dev/uses-webp-images/
Basically, it is to reduce the size in kb of the images. Since these image formats provide better compression and quality for images, using them will reduce the size of your images, and that will have a direct impact on performance because less data will need to be transferred to the user’s device.
How to fix this?
This conversion has to be done outside WP Rocket, as we cannot convert your images to WebP or to any other format.
To achieve this, you can easily optimize your images with our image optimization tool Imagify and its WordPress Plugin: https://wordpress.org/plugins/imagify/