As far as speed goes, there doesn't seem to be a whole lot of difference between formats from my experience. The image size and how much re-scaling the reader has to do plays a bigger role.
As Dale wrote PNG or GIF is best for Graphics or Charts since it's lossless. (Unlike JPG, which is lossy, i.e. drops or modifies image information in order to achieve a small filesize)
You can currently scribble on images, even template them in the PC software, but it won't merge them yet. This only seems to work only for PDFs for now, and not very well at that.
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https://www.mobileread.com/forums/showthread.php?t=30465)