The maximum displayed size of an image in a MOBI ebook on the Kindle 1 is 525x640 (the K2 is the same or similar), see
Kindle Automatically Doubles Image Sizes. However, you can zoom to full size (K1 or K2), see
Kindle now Zooms AZW Images. Another informative thread is
Image optimization for Kindle 2.
One difference between the K1 and the K2 is that the MOBI/AZW cover image on the K2 is 600x800, vs 525x640 (like interior images) on the K1.
Note that these are all about the displayed size. If your original images are GIF then they might get reduced in size to fit in about 62KB per image, although I think it is now more usual for them to be converted to JPEG (possibly loosing quality but maintaining the image dimensions).