I don’t know if you can tell, but those pictures are perfectly level, and they have stayed that way since the day I hung them up. That’s right, I haven’t had to straighten them once!
My secret? After you have hung up your picture on the appropriate nail/hook/whatever you want to use, put some sticky tack on the bottom two corners and press is into the wall. It stays perfectly straight and you’re not always having to fix your pictures!
Image from amazon.com