Now before we launch into the meaning of the thing, a little background info. Dostoevsky saw this painting when traveling in Switzerland, and was so struck by it that he climbed up on a chair in order to look at it more closely. (We have a nice little story about this in his wife's diary, where she's all stressed that security would make them leave.) Okay, so why does Rogozhin have a copy of this painting hanging in his house? And why does he constantly stare at it?
According to Dostoevsky's own notes, and then according to Myshkin and to Ippolit, this painting is pretty much right at the crux of Christian doctrine. Central to the Christian faith is the belief in Jesus's resurrection. But if you look at this image of what Jesus might have looked like after his crucifixion—beat up, bloody, emaciated, laid out on a slab, and really very, very dead (Holbein used a drowned body as a model)—you're forced to come to the conclusion that Jesus was just a mortal guy and that his resurrection is completely physically impossible.
For example, check out Rogozhin's own admission of the effect it's been having on him:
Over the door, however, there was one of strange and rather striking shape; it was six or seven feet in length, and not more than a foot in height. It represented the Saviour just taken from the cross.
The prince glanced at it, but took no further notice. He moved on hastily, as though anxious to get out of the house. But Rogozhin suddenly stopped underneath the picture. […]
"I like looking at that picture," muttered Rogozhin, not noticing, apparently, that the prince had not answered his question.
"That picture! That picture!" cried Myshkin, struck by a sudden idea. "Why, a man's faith might be ruined by looking at that picture!"
"So it is!" said Rogozhin, unexpectedly.
just sharing because one of her favorite books is the idiot and she often talks about christian symbolisms, cos she was raised as a christian