May contains spoilers for volume 4:

Involving the painting by Madame Maria Stanford. After Cain uncovers the picture underneath, it's a picture of a woman... doing something. Cain thought, "She's... No, that's horrible...!!" but they never explicitly said what was wrong with the picture, and I still can't figure out what was wrong with the picture! If you get it, please explain it to me...

Um...maybe she raped her son or something? She was an abnormally possessive mother (if I remember correctly), and her son obviously hated her. This is only speculation on my part, though; the story was pretty hazy about the details of that painting so I don't think we'll ever be able to say for sure.

This thread is a few months old but, I keep seeing this question in places again and again. So I hope no one minds me answering.

You need the Japanese version of Volume 4 (but be warned it is potentially very offensive). Because it is actually very clear in the original what the painting is. The VIZ version has censored art and dialogue (including the painting) in several places throughout the three Bloody Maria chapters, all in relation to the religious content in the story. At least the first printing was that way and I bought the VIZ version years after it was first released. (So I'm very glad I bought the Japanese version first.)

The picture is of:


And a little more FYI, the French version is not censored and I recommend it since I think the translation is excellent, though the publisher has stopped printing the series now.

Oh my gosh!! Wow, THAT explains it, thanks for clearing that up!!