I know there are alot of images on this site, but I really think JPEG was a poor choice. The compression artifacts on UI elements look especially bad. I also agree with philwebster about it being really hard to see the full image.
Oh that's too bad; I didn't realize that. It looks like Apple's Enterprise Partner Relational Feed serves up JPEGs as well. I guess the last place to check would be by monitoring network traffic coming out of the iOS App Store app or iTunes Mac app, but that'd be a bit of a pain (not even sure those use PNG).
