I've discovered something else significant - which is how to break it!
If I install other themes, then the cover art stops working. So if you've done the same, then try what I suggest above: remove the .rockbox folder completely, and replace it with the current build ONLY. Test if it is working BEFORE adding any extras such as themes.
I haven't had time to investigate exactly why that happens and whether it can be worked around (or perhaps I'm doing something wrong when I install the themes), but I'll try do do so at the weekend.