Your best bet is going to be contacting local artists or the restaurant’s management. If the restaurant opened in the past few years, someone will remember who bought the art or where it was from. You might have luck on a local Lemmy community, but I doubt you’d have much luck, since most are pretty small and any given city probably has thousands of artists (assuming the person is even still active or that they were local to begin with).
You could also try reverse image searches online.