|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:
Spoiler (mouse over to view)
The Virgin Mary strangling a baby Jesus. And Cain says instead "Maria (the Virgin Mary) ... killing her son...!!"
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.