I dont know about the ipad?
But there is some mobile devices and pads that screws images upp if its larger then 512x512.
When doing your images try to have the same size in width and height.
I remember this from my experiments in android java as it whas reported alot there form other devs.
Its the more powerfull and newer devices that you can use larger textures on.
Some devices where larger textures and images worked on became extremely slow when using images larger then 512x512.
cheers.