The contentWrapper image is generated automatically based on any changes you make to the layout. If you want to change the background image, on the Layout Tab make sure to un-check the equal column lengths check box.
Ok. I have not delved into it yet, but how is the new CSS Sculptor creating equal column lengths? I did have that box checked. What I noted was that it was fine as long as the left and right columns were taller than the content, but if the content was taller than the side columns, the side column borders still stopped early. I wanted to edit the contentWrapper background to use faux columns to overcome this. If possible, I would like to see this checkbox ensure that all columns are equal height within the contentWrapper. Right now, I have already moved on with this site. I justed edited content wrapper after the page was created, but obviously that isn't the goal of the extension.
Well I can try to recreate it. I started a 3 column centered fixed page and began to customize it. I didn't finish but saved the partially complete style. I came back to edit is and wanted to change from 940px to 950px and could not. It may only happen after you save and then come back to edit the saved layout.