I just realized that if C2 is going to be making web games/web sites, the ability to use jpegs for backgrounds and such is important to reduce file size and page loading times. Hopefully C2 will have some way to use both jpgs and pngs? Or does HTML5 canvas require everything be png?
Canvas can do JPEG. I'll add it to my todo list. They would just be converted to JPEG on export - in the project they'd still be PNG, because they're lossless. (You don't want the quality to degrade every time you click 'Edit'!)