Which MCP are you using? In the versions before iOS 6, there was a toggle under Task Controls for "Old Background". That one actually drew from your folder switcher background image, so it was pretty easily themed just by changing the folder switcher linen file, clearing the cache if needed, respringing & toggling that switch.
But--that function isn't present in MCP 6+. I haven't yet found how to theme the background of the new one (admittedly, I'm pretty used to the translucent drop down now and haven't looked that hard).
You can get a list of all files installed and their locations with a given tweak via AppInfo (from Cydia). If he's not simply repurposing an existing UI image, it should be in that list. I'll dig around a bit more too and see if I can find it.