Background image on pages and posts

Hi all, i love this theme. Im just beggining with creating websites… So im stuck at soem point, and i dont know where to look anymore.
The problem is, i instal plugins for changing the background on new pages, besides the homepage (there is everything normal and easy…), and none of them works like it should. I want to set an image on all that white background when u open an new page. Is that possible? Or if not, then can i change the colors a bit? This way it dosnt look good…
I hope someone can help me with this.