Some website speed testers currently list among their recommendations that we use ‘next generation’ image formats. There may come a time when this is a good idea, but that time hasn’t arrived.
What are ‘next generation’ image formats? Primarily they are JPEG 2000, JPEG XR, and WebP. The first two are better versions of the ubiquitous JPEG, designed for specific applications but giving better compression. WebP is an excellent, open-source image format which is smaller than JPEGs and PNGs but can handle transparency and animation too. Unfortunately there’s a major problem: not all browsers support them; most current ones may now do, but there will be older browsers out there for a long time. So it would be foolish to choose these formats for everyday web use.
There are ways of serving up next generation image formats to browsers that support them and providing fall-back traditional formats for others, but that multiplies the effort required to supply images in the first place. Faster-loading websites are something we should all be looking to provide, but using next generation image formats may not be a cost-effective strategy at the moment.