I don't understand?

When Jack shows Rose the picture, we don't see her at all. But if you watch it, you'll see when Rose sees it that she'll tilt her head to the side and she says: "Ohhh...Haha." Jack starts laughing and says: "Aw, she had a good sense of humour, though." por Rose's facial expression and how she reacts in general, I think he was dead serious. But that's just my opinion.

I thing he was being serious that she was one legged. I take it literally, but good question

Yeah, she was literally one-legged. If you pause the movie a little before that point and put it on "slow" or whatever, right before Rose flips the page, you can see the drawing of the one-legged one lol. I heard in the commentary of the movie that they tried to cut that drawing out of the movie because they thought people would come up with a mais interesting picture in their imaginations lol.