There seems to be a lot of confusion in this thread. All that's being asked for (if I understand) is convert to JPEG on export to reduce the download size for certain images. Of course JPEG is lossy and doesn't support alpha, which is why it is never used in the editor, it just could be an option to further compress JPEG suitable images when exporting.
This has been on my todo list for a while. A workaround in the mean time is to convert images to .jpg yourself after exporting, then you need to update the filenames in two places: first in c2runtime.js and then also in offline.appcache. (find and replace myfile.png with myfile.jpg in both)