Hey fellow caker! (waving madly)
I have been having the same problem. Over the last few months a relative of one of my in-laws has been posting pics of cakes on my wall with similar notes... most recently a picture of a dog cake, that while very cute was definitely decorated by a novice; and one of a very, very realistic cockroach (very well done technically, but I found it disturbing). I don't care to have anyone thinking I made either of these cakes, so I have immediately untagged them.
I don't think you have any obligation to respond to the poster. If others have commented on them, I would be honest and say, "Actually, not my work. A friend thought I might like to see it."
I'm trying to figure out whether or not to ask the poster to stop, and if doing so will cause tension in the family. She is really a lovely woman.