Turn the picture onto its front. Cut off the string at the back.
Measure the backing between each loop that the string was tied to. Add 2 inches, and cut the length from a spool of fishing line.
Tie one end of the fishing line to an end loop on the backing. Tie the other end to the other end loop on the backing.
Hammer a hanging nail into the wall where you want the top middle of the photo to be. Slide the photo over the hanging nail's hook.
Rest the fishing line on the hanging nail's hook. Adjust the picture so that it is straight.