Okay, I determined that if I go into the "Create and Name Devices" screen, I can drag a new graphic over to the device. Sometimes a dialog box will pop up asking if I want to overwrite the entire device or just the image. That works for some of the devices/actions, but for some of them (especially the ones I'm interested in with macros), the new graphic just pops in and overwrites the entire device. The dialog box asking if you want to overwrite everything or just the image never pops up. I tested it with a couple of them, and two of them are almost entirely identical (one to watch/use Net Radio one to watch/use the Wii) and on one the dialog comes up, the other it doesn't.

Any ideas as to why?