The picture of the woman?

This was a GREAT film (really confused by people who hate it) but one thing I was never able to get was the picture of the woman and the woman in it? Who was she and what was the purpose of Marcelo's mother having it on her mirror with all the rest of the pics and eventually unfolding it to reveal the other two men in it at the end?

I get that he saw that threesome going on during Carnival, but I don't understand how his mom wound up with a picture of it?

"Can ya give good fairy tale?"