Take your picture and put painter's tape across the back of it where the hangers are.
(I hate how this sentence is worded, but I've re-written it 10 times and am giving up. I hope, even through the poorly written structure, you know what I mean)
Mark where your nail will go.
Then peel your tape off your picture and put it on the wall using a level to make sure it is straight (you can also mark the middle of the tape between the marks to determine the middle of the picture). Then just nail (or screw) where you made your marks on your tape and peel off!
Easy as pie!