Well, in worst case scenario he can split the image into multiple pieces: 2048x1024 + 2048x64.
As zork said, you can't use PNGs, but you can use TGAs and BLPs, I use TGAs, most editors support export to TGA, moreover, unlike BLP TGA doesn't have artefact issues.
You will have some issues w/ PS and TGAs, Adobe really don't want to add auto alpha mask generation, so you'll have to either make your own, but it may be a PITA if you have really complex texture w/ semi-transparent objects of various colours, or save your image as PNG and later convert it to TGA
