Subsite Theme - upload background image default location

This is a follow up on the upload location issue that I had with the Zwin theme. See source:

The Zwin theme was abandoned by its creator and was not really Wordpress compatible. So far aristath helped with the above issue by simply rewriting some codes, making a step closer to be Multi site compatible.

One last request:
In the theme there is a option to upload a custom background image, unfortunately this is written away in the theme files itself instead of the user's/subsite upload folder. Making thing worse, background images can be deleted
from the theme as well, including the default ones.
How can this be changed? @aristath would you be able to assist in this one as well? It would mean a lot and then this nightmare is for what I know, over!