I couldn't find the exact documentation you were referring to, but there were some documents explaining faux columns. It is easy to simply replace the image in the website files, but I was wondering if the documents also advocate replacing the images as part of the files the go with the preset in order to have the new preset complete.
The method you describe is ingenious in creating faux columns. But it needs a preset design to start with, so since I always start with one of the WA presets, the slice it creates comes from that design. And I do understand your explanation of what occurs when the Equal Columns checkbox is checked, and why it happens.
What I'd like to know is, why then is the contentWrapper background image uneditable from within the wizard? Is it a technical reason or is it to ensure the user doesn't insert an incompatible image and really mess up the design?
What if we did the workaround another way - leaving the checkbox unchecked, inserting the background image, creating the page - then add the necessary code to the page and or stylesheet?