MobiPocket's documentation indicates that large images will be resized. I confirmed that this was the case if you made the window very small on a PC, but I don't have a PDA to test on. See
Images in MobiPocket.
A related issue is which devices support JPEG images in MOBI files, since these are typically needed to get large images. The "StandMars" PRC files in the above thread should work on the Kindle (mine arrives tomorrow), and can test MobiPocket's PDA reader software for JPEG support.