Background Image-Twenty Twelve

Twenty Twelve Theme is not letting me use a full screen background image! Even though I’ve installed “Full Screen BG Image” plugin! I’m only seeing the desired image at top of content. This is not enough! I want to see the background image at sides of the content also! For some reason, I cannot even customize the background image through; Dashboard/Appearance/Background…It will not let me Tile, or Position or select Scroll/Fixed. What’s going on!!??
Desperate for help! Thank you! Jamie.