It depends on the image. Solid blocks of color and PNG would probably win. The very worst thing for all images is random noise followed by thin lines ! If you have a door in an old school map then its better being solid rect than an open box for example. Also, lines that are horizontal and vertical will be much better than ones at an angle. With JPG its a bit more complicated knowing what shapes would compress better as all the compression isl done in frequency space. JPG will have about 16K minimum size even if its all white but PNG would be a few hundred bytes. Also, ensure you save your PNG with specific number of colors 1, 16, or full. If you have < 16 colors then don't save full color as its hugely more expensive.