Do you ever get tired of looking at art that doesn’t mean anything? The type of art that has no interpretation, because it is so straightforward — art that is meant only to be pretty, meant to grace the walls of restaurant bathrooms? Occasionally Rachel Harmon feels surrounded by works like this and needs art that has meaning to remind her of the wonderful power of art. That’s how she found Renée Cox’s “Yo Mama’s Last Supper.”