It would be nice to be able to back up thumbs, jobs, raw cache, etc. So, what folders can I sensibly backup? Would it then be simply a matter of substituting the back up folders (or their contents) for the (empty) folders in a fresh Qimage installation?
You have it right!
I don't know which Windows you have, but typically you should find a Qimage folder under C:\Programdata\ddisoftware\Qimage.
Then save all the subfolders.
See attached screen smap.
Fred