Why not just fire up photoshop (or The Gimp! for a free alternative) and save the image as a PNG yourself?

Then you control filesize.

As to why; there are many advantages to a PNG, most of which is that it's an almost lossless compression. Also, it's what's used all through lightstep because it loads as fast as a bmp and it supports real transparency. :)

Try it, you might just like it instead of jpg