Good references is one of the keys to good art, but images can take up a lot of disk spaces and eventually you’ll want to look for ways to reduce them. In my search, I came across two formats: WebP and BPG.
As of this writing, BPG is the best overall, but it’s not widely used / supported because of its issue with licensing. digiKam can’t view BPG file(s), and there’s no easy way to convert images to BPG format for those on Mac and Linux. WebP on the other hand has good support and digiKam is able to view it, but can’t convert to it. For batch conversion, XnConvert can get the job done.