In general, it looks like other cities do public art on bus shelters mostly using vinyl wraps, some use printed posters in the advertising panel, and a few have direct-painted murals or artist-designed shelters. 

This one from Arlington, Virginia, looks like what we were talking about with wraps.  I really like that they can be translucent for safety, too.
http://www.arlingtonpublicart.org/bus-stop-art

It looks like Milwaukee does a similar type concept, and the images and diversity of content look great!
https://www.ridemcts.com/programs/bus-shelter-art-project

Here are some images from a partnership between the transit authority and a school district with vinyl wrap using student’s images in Ontario, Canada:
https://www.facebook.com/pg/Limestone-District-School-Board-308623265872996/photos/?tab=album&album_id=2223018127766824

I’ve attached a call for artists from Raleigh that I thought was particularly helpful and has lots the details for the dimensions, locations, etc.   

It looks like this volunteer-based program in Seattle is much less expensive to operate.  It also looks like most of the ones in Seattle are actually painted on the surface, so you could ask them about cost, cleaning, and maintenance.
http://metro.kingcounty.gov/prog/sheltermural/

Hope this helps!  

Best wishes,

Sean