Problems with the background

Hi to all, I have a huge problem about the background of my forum
First of all i'm the admin of this forum http://villielain.forumotion.com/forum.htm and I have a problem:
Like background i want to put a phot but doesn't goes. I explain: During the preview the photo like background look but after I have clicked "Save" I see the forum properly the forum looks like before I made the change.
Please if someone help me thanks so much!