Background conspiracy

I have been trying to change my blog background for like the last 3 weeks at least! For some reason, it doesn't matter what background I pick, or from where, it will not change! I am really getting frustrated! When I originally load my page, I will see the background that I have tried to install, then it changes to the old one. If anyone can help, please let me know, because I am so not happy right now! Please, please, please help! I may end up hurting the computer if I can't figure this out!