When using the Advanced Editor (IE9) I can put a background picture on a story page from Source with HTML & CSS, then see it when going back to Text. But when looking at the story in the Story Viewer, The background is not being shown.
I can also put a background in a Forum Post in the same manner - Put in Source and view in Text. But if you post it, not only does the background not show in the post - but it actually changes the background in the white space on the Site itself. I did a test and changed all the white space on the Site (from my PC at least), to bright green via a Forum post.