In Calibre there is a MOBI Output option for "Rescale Images for Palm Devices". I'm not exactly sure what it does, but it is worth trying. The ebook must be DRM-free, but there are DRM-stripping Calibre plugins available if necessary.
In older MOBI ebooks (almost always in .prc files) there was a ~64KB limit on images, which was caused by the single record length limit in Palm file formats. Recent MOBIs can have images span multiple records. They also support JPEG. Finally, PRC/MOBI had options for providing a small (alternative) image for small screens. So Calibre is doing one or more of these things to optimize for Palm Devices.
Last edited by wallcraft; 01-19-2011 at 08:12 PM.