There are lots of ways to hang a picture, this is one way I found was helpful in knowing where to put the nails.  Take a small strip of duct tape and roll it up sticky side out.  Place it on the back of your frame so that the place for the nail is lined up right under the tape. 
All you have to do is hold the frame up where you want it to be on the wall, then press on the tape that sticks out above the frame so it sticks to the wall.  When you set the frame down the tape will be in the spot on the wall and all you have to do is put a nail at the bottom of the tape!

Easy cheesy!