The use of a bull can be seen in prehistoric art (as you mentioned, the cave paintings), but there's no reason that it couldn't have arisen independently each time bulls were introduced. If the image came from the mainland, it seems likely that it came from Anatolia, along with the people who brought it.
Perhaps Anatolia, but if you look at my Wiki post, you will see they are using Celt-Iberian bull stock in some of their rebreeding efforts.