My tumblr won't let me upload an image as a background. What to do?

I’ve tried all the other solutions that were posted in response to questions similar to mine, but it still won’t work.
I don’t think it’s my theme, because I saw someone with the exact same theme, with a different background image.
I want to use this image—> http://s270.photobucket.com/albums/jj105/swolleneyeballs/Photography/Ones%20I%20Like%20Most/?action=view&current=LostArtifact.jpg
as my background, but it ends up flashing once, then disappearing, and then my background is just black.

Am I doing something wrong ?
help.