For transparency on EoFF you should be using PNG-8 or GIF. PNG-8 has slightly better compression last I checked for much the same quality.
GIF is used for animations, duh.
JPEG is used for just about anything that doesn't use transparency(it generally has a better color palette and can be set up to fit within just about any size limit/quality).
PNG-24 is used for complex transparencies that look bad on GIF(you can build around it if you know GIF's limitations but for most people complex transparency is best in PNG-24). However, good luck getting PNG-24 images in under our size restraints. It isn't all that difficult to end up with a small signature with a simple transparency that is 50+kb size. And I have seen PNG-24 signatures that width and height wise would fit here that possess 300 or more kb in size.
PNG-24 is also used for any image if size doesn't matter. It is a standardized loseless file type that is readable in practically any software(unlike many loseless file types).