People are creative, as evidenced by this art work and a national flag design (thanks Lemo!). It would be so much easier to create cool 2 dimensional art work if the shelves were invisible and shadowless.
Please make shelves that you can put stuff on that will blend invisibly into the wallpaper. This goes a long way to making player designed art a reality , in a relatively simple way. This should be free for at least one wallpaper, such as the light blue stucco in Fae's example.
The only tricky part is making the shelves visible enough to put things on in the first place. Perhaps a slight outline in decoration mode, but not in regular mode? I dunno, I'm just an ideas man.
ETA: You can stage everything with a different backround where the shelves are visible (like a dark background give the light blue stucco example), then switch to the stealth background when you're done. Yup.