OS X :: Setting Permanent Background For Guest Account?

I rent out a cabin and I would like the guest account that renters use to have a permanent background. Is this possible? What I am trying to do is allow renters to upload their photos to a shared folder (already set up). This shared folder would be a compendium of photos that renters have uploaded, and the background would change every 5 seconds to show off all the great pictures. Possible?

The next avenue I will explore is instead of using a guest account just creating a normal account for renters, but for obvious reasons I am wary of doing this