Well, being a professional graphic designer I understand very well the difference between these formats and I must agree with HotGod that would be really handy if C2 could deal with JPGs besides just PNGs. The download time for intensive graphics games could be dramatically reduced.
Indexed PNGs may seem like a good solution but they are only more efficient than JPEGs when the image has few details and lots of flat colors. When an image has lots of gradients and details a 24-bit PNG will be a lot heavier, and an indexed PNG yields poor results due to compression caused by the limited palette.
The benefits of JPEG are not related with video memory. Consider the amount of hosting space, download time, and the user's disk space after downloading. Think that a lot of potential audience for web games have poor internet connections, or consider the emerging mobile market where the devices have low storage space.
PNGs and JPGS are complementary RGB raster formats. Both have their strengths and weakness, but alone they don't cover all the ground. Supporting both would be a wise decision, specially for an application that is headed at web content.
I wish C2 could load separated JPGs for the image and the alpha channel, then would be feasible to use large images with transparency. This way the alpha could have a very high compression, and not add a lot to the size of the image like an 24-bit PNG does.