Looking in http://maemo.gitorious.org/fremantle...round.c#line58 and http://maemo.gitorious.org/fremantle...ound.c#line245 it looks like the two folders it looks in are indeed /usr/share/backgrounds and /home/user/MyDocs/.images and it would need someone to change the source and recompile to change that. I'm not sure if you'll be able to persuade Nokia to do that, but there's at least one community-compiled Hildon Desktop out there that might be more amenable to this kind of change.