png's can definitely be opened in Photoshop...I'm not sure exactly what your doing but you need to create a 512x512 .png file named loadmap and put it in the info folder of your map.
And can someone double check the size for me, not sure if its the right one.
I have the same problem - I have PS CS3 and it wont open any of the PR load backgrounds.
Iranview will open it, but it brings up a warning that it is actually a .dds file, with an incorrect extention, so it automatically renames the file to a .dds open it.
( and yes I have all the .dds plug ins etc installed )