Reference librarians are very accommodating, but I don't think I could ask the Penn reference librarian to go there and take a snapshot of it for me. (Although you never know, and I admit that I haven't actually tried).
However, there is no reason why someone who's already there couldn't take a picture and upload it, and use that as the source. (I don't know what the copyright is on walls, but it should be fair use if it isn't free)