Images come in different formats.
The most widely used format is
jpeg. Another format is
jpeg gives better size comparison,
png lacks in that department, but
png has transparency.
jpeg gives better compression format but does not allow to use transparent pixels.
If an Image does not need transparency it should be in
jpeg, but if it requires alpha it need to be in
WebP is a new image format that offers an interesting ground, it has compression ratios similar if not better in some cases than
jpeg, but also allows to have also transparency supported by
png. Therefore this new image formats provides good compression and alpha transparency where needed.