I had real problems with this at first. Firstly, I had a few problems with the correct path, as I took the "plat" in the Attributes help message literally; it is actually "plaf", but that was less of a problem as relative paths are fine. Secondly, there needs to be a default icon for the Actor or it will not appear, which is more of a problem as I couldn't work out why Safari was telling me that the graphic was loaded successfully, but the actor wasn't appearing.
I'm going to jump in now and see if I can work out how to get a Background modifiable this way, and if it is possible to load a new sprite during run-time, rather than at set up, but if you have any thoughts on how to do this (to save me time) I would appreciate it.