Hi BTPS,
I know exactly what you are referring to. I use the same method for creating my basic templates, so I had to figure out a quick work-around when my SD account transferred to IF. IF has an image position migrating set-up similar to Auctiva.
What I do to make my personal background picker work from within my template, is put a 1 pixel clear gif in each of the image slots I'm not using for that listing. It acts as a placeholder to allow me to use 13-16 for backgrounds, buttons, or whatever. This way my template coding still works. (I just uploaded some clear gifs, named them and pick them just like a regular image.)
I have not tried this with Auctiva yet since I'm just beginning to make this switch, but it should work.
I hope this helps, and also hope that it works with Auctiva because this is my favorite method for customizing my templates.
PJ