I'm no artist, but if I wanted a very high quality image I would use bitmap (bmp) with full compression/quality (or whatever it is)
I usually use PNG.
Though if I had to choose out of JPG or GIF, I would use GIF.
For both though, make sure the quality is as high as possible or you will/may get noticeable quality loss.
* Edit: Bitmaps are usually larger in file size.