Back in the olden days (shortly after NR2003 came out) people began releasing stuff in .psp only (probably because it was their program of choice) and, at the time, didn't realize/know that PS (Photoshop) can't/doesn't/won't open .psp files. PSP on the other hand, not being the gigantic, snooty, holier-than-thou corporation (Adobe), made their product more user-friendly by enabling it to open pretty much any image file known to man.
As I said, there are some real good deals to be had out there. I picked up PSP8 something like 6-7 years ago on eBay for $25- just so I could 'Open' and then 'Save As...' the above mentioned files. Probably even cheaper nowadays.
As for tutorials, the best one I know of is the one by JimmyK. It can be found ->
Here <-. Even though it was written for the original game's templates, the processes remain the same. If you go back to JimmyK's home page there's an option to download the whole tutorial as a Word document.