Some sublimal pictures are there indeed, however as rem already pointed out:
At most you could prove that some artists were just having a little fun.
It is very well known that the artists doing paintings in the middle ages had to do them anonymously, because of pressure from the church. To make a painting still theirs they hid little jokes or clues in them. As the society demands the same from it's artists it is to be expected that they also find other ways to 'sign' their work. And what better way then to hide something that really shouldn't be there?
Greven