I thought the wife was being a bit of a SS to draw such a hard line in the sand over one little picture. It isn't like the husband goes around redecorating the entire house just for his mother. In Prudence's response she says "Most telling, perhaps, is not your mother's views on pornography but your implicit description that unless everything goes her way and everyone tiptoes around her, she ensures a miserable time is had by all..." I think Prudence is engaging in a bit of an interesting assumption. He didn't describe her as demanding and pouty -- just that this one issue of nudity is very bothersome for her. She could be the pillar of gracious guest otherwise.
But it's funny in a ironic way that the husband married someone who is so insistent that he toe her line that she's willing to cause him problems with his mother just to make point that, "this is her space and if someone doesn't like the picture, that's their problem." This is his house too, so it's not just her space. I wonder if he makes any sacrifices to his time/schedule/preferences when her family or friends visit. If so, and who doesn't when guests visit, it's her turn.
I have many decorative pieces that I adore, but none so much that I'd risk alienating either my (hypothetical) spouse or his family just to mark my territory. It may sound like his mother's comfort is more important that his wife's, but isn't the wife making a statement that this photo is more important to her than her husband's happiness and relationship with his mother? Seriously, she can't live without it on the wall of the bathroom for a few days? If the photo was so key to her and her husband's identity, wouldn't it be on the living room wall or their bedroom? IMO, the damp conditions of a bathroom is where people hang the least important artwork in the house.