This issue has been
'fixed' now in
Mobi2IMP 9.2 so you can convert once and build twice without any image problems caused by the eBook Publisher's weak handling of image re-sizing.
Basically, the problem, as discovered by zelda_pinwheel, is that for images taller than wider, the aspect ratio is not being retained when ebook Publisher automatically re-sizes images larger than the device's usable dimensions.
Experimentation revealed that as long as the proper aspect ratio was used, any larger than screen images would NOT exhibit this re-sizing problem.
SOLUTION: Just pad horizontally with 'white' background enough to attain the proper aspect ratio and then re-save that new image. No images are altered, cropped or chopped, just padded evenly horizontally!