Legal Graffiti is an Art Form in Downtown Billings

If I said Beetlejuice, what comes to mind? Say it two more times and see what happens?

The Graffiti in downtown Billings is absolutely amazing.  Who's responsible for it?  Is there more in other areas of town?  Love that this city appreciates art in all forms and not just the traditional idea of it.  Is this an organized group and where can I see more of it?