The reason is that Apple only allows graphics with a maximum edge length of 4096px (height and width), see http://iosres.com/ for more information. It's also the case that the edge lengths of 2048px (height and width) wants crash older devices.
We recommend formatting images so that they have less than 2048px edge length. Or you can break the pictures up and put them back together in InDesign or Purple DS Composer. This avoids the problem of maximum edge length and, if necessary, displays larger graphics.