The background image is a png resource in shsxs.dll. It can be changed, however there will be a big band of green color in the center of the start screen. I'm guessing there's a registry entry that needs to be changed. I haven't found anything yet, it might be in an xml file in shsxs.dll actually.
That "green" color start screen background can be changed (not in the developer preview). But using some code tweaks, you can change it in the present build (WDP 8012).
Rafael Rivera from WithinWindows says he will write up a way to change the color soon. So, just wait (by the way his site is down as well).