I'll have to agree with Kirby here. It's be best if you were to keep it simple, limiting it only to what it should have. Some handy things to have:
A. Make the bar at the top show the filename, the size of the file and current zoom(in percentage)
B. A "Back to original size" button or the like.
C. Rotation functions.
D. "Previous" and "Next" buttons, in case you want to see a folder full of pictures or the like.
Other than those(and some other small things you can think of), I don't think much else would be needed or worth it. Quality over quantity. Keeping it as friendly as possible towards the memory would help the program.
Just my two cents(given Squall, these are two cents he'll quickly discard).