The Cobra Tack comes in a variety of sizes. I found mine at Lowes. They are simple to use and it really leaves no guess work to hanging pictures. Autumn recommends this to all her clients. It is a great way to hang a grouping of pictures. It keeps the picture level and makes it easy to hang equal distances from each other. Click on her name above to see her tutorial!
I tried them out today on 5 different problem pictures. They were so easy.
I hope this offers a solution to all your crooked pictures!