From two years ago, see
A Comparison of format limitations based on The Hobbit. It is still the case that, typically, an ePub will have higher resolution images than MOBI/AZW. However, the Kindle has about the best image handling support of any dedicated reading device.
As whitearrow says, images in ebooks are of highly variable quality. If I think images will be important, I prefer to buy ePub and I would do so even if I was reading on a Kindle. Calibre's format conversion can target an actual device and shrink images if they are bigger than the screen (the Kindle will do this for you, but resizing off-line is more efficient).