The explanation makes perfect sense. I can also observe theneffect in Dreamweaver when the div containers are being highlighted.
So, as I understand it, the way the extension works - the act of checking the box merely assigns a Pre-made background image from the chosen theme and makes it unchangeable in the design tab?
Is there a reason it's unchangeable at this point, do you feel it would create more problem than it would solve if the user inserted an image that wasn't the right size or design?