I already had this problem with the previous version of worpdress and I solved it by deleting some lines of code, but I can't remember which.
The problem is with the background.
When the "beta resize" is off, it moves when I resize the window. To be as clear as possible, if I make the window smaller, the BG will become smaller to match the window (except that it will create blank sides because the ratio is not to good one).
And when I take back the window to its original size, the picture will keep the small size.
The goal here is to have a "normal" background, that doesnt move when I resize. But the plugin won't let me do that and I can't remember how I corrected it last time.
Hope it was clear enought!