The maximum image size in a MOBI file on the Kindle appears to be 640 by 520. I enclose an example set of images in a MOBI file, and Kindle screenshots. The 6 images are:
The original is 404x618 and normally the aspect ratio of the image would be conserved, but here I deliberately stretched it to the other listed sizes.
Note that the first five images are the same height (640 pixels) when displayed on the Kindle, which means that the 404x618 image has actually been rescaled to be 640 high. The 525x700 image has definitely been reduced in size. The last image is less high, because 540 pixels is rescaled to 520 which reduces the hieght to maintain the aspect ratio.
The .zip file contains the six images (last 5 generated from the 1st using IrfanView), a HTML wrapper and a .mobi file created with html2mobi.