Actually, unless you need serious edits (i.e. edits that you want to incorporate for real) from the other members, JPEG is the best choice considering the painting style of the backgrounds and it could save up a lot of space (though I agree that the compress quality could be tuned up a bit to make them less blurry, or, maybe it's the photograph process or they're saved as 320x200 and then scaled again up to 640x400). For the character, definitely don't use JPEG. Just save as PNG in its original size and use the width tag as mentioned by KhrisMUC.
Personally I'll say they don't look that much like Sierra style, but I'll say your paintings have a certain charm in them, so I pretty much like them.