I'm not sure about the gen 3 games, but in future generations (namely gen 5) I have found that your Union Room sprite depends on the trainer class selected on your Trainer Card.
Edit: I just checked Bulbapedia. It doesn't mention how the trainer class is selected for generation 3 games. I also checked Serebii. It doesn't mention it either... Whatever the case, it seems selecting it on your Trainer Card wasn't a feature until Gen 5.