The room map is the layout of an entity and allows you to move around from room to room. As for using it, there should be some blue highlighted squares on some of them and this indicates the rooms you can move to, while the red cross indicates where you are.
As for custom room images, they are a little wonky at the moment. They used to have a reasonably sized image on there you could see, but that was lost a while back. There is some talk about making the 100x100 image you see in the room a custom image. Either way I do not believe there is a way to get a custom image at this times.