Apple has added thousands of pixels to its image size allowance, jumping to 4 million from 3.2 million. That's good news, since Amazon's upper limit has been (potentially) larger for a while.
Current Amazon Guidelines (version 2014.3) don't speak in pixels, but in image file size. The max file size for an image headed to a Mobi is 5 MB (with a total Mobi file size of 650 MB, so be sure to tally your image sizes as you assemble large projects).
It's certainly possible to have a 5MB image that measures more than 4 million pixels, so it's up to designers and developers to create a workflow to use the highest-resolution image in each platform.
Note that Apple's file size limit remains at 2GB, so be careful when sizing images up.