the tutorial i was referring to is "Creating Fixed Faux Columns" in the archived documentation section of the CSS Sculptor support page.
the content wrapper image is not editable within the wizard, because it is created by the wizard, I'm not sure what would be gained by making it editable.
I version 1, it did not create the image automatically and you needed to follow the tutorial to apply it manually. Many people had difficulty with this, so the process was automated.
you can certainly do it manually after the design is created if you prefer.