TheIndustrialCottage.blogspot.com has an ingenious way to get your picture hung "just so" without marking up your wall. Check our her post HERE. What a little smartie pants! :)
Put a little dab of toothpaste right on the end of picture hook.
Take your picture and hold it up where you want it without touching the wall. I do much better with eyeballing it. Once it is were you want it, gently tap the frame against the wall. You will leave a little toothpaste mark on the wall.
Hammer your nail right were your toothpaste mark is.
Wipe the toothpaste off the wall.
Wipe the toothpaste off the frame.