This might only work in the Buy/Build mode (not sure about live mode):
Click and hold the painting/photo/wall lamp etc. When you hold it you can move it up or down. This way you can have multiple wall shelfs on the same wall etc. Using Moveobjects on cheat might help to put them even closer together.
I tried moving the paintings my sim had painted herself (using the easel) and the skill certificates, but I think their height could not be changed.