You can use kgPanels (sorry, too lazy to search for a link, it's here, anyway) to display textures in the game.
There are only some rules for those textures (without being sure this list is complete):
- texture-sizes are power of 2 (2, 4, 8, ..., 64, 128, 256) in both directions, but not required to be square (so 64 X 256 would be ok)
- save them as 32bit TGA with an Alpha-Channel (simple in PS, just add the apropriate Alpha)
- just put them into a folder, which can be accessed from within WoW, some folder inside of AddOns should work.