Ultrasounds totally squick me out, as I don't want pictures of my own uterus, much less someone else's uterus. In addition, the few that I've seen have come with a lot of pressure in the form of expectant parents saying "Do you see that? There's the foot, can you see it?" and me having to say "No, I don't", so I have bad associations.
I would be a little grossed out by the card, and would try very hard to keep it showing one side and not the other. But, since our Christmas cards include pictures of our dog, and lots of people think dogs aren't family, I don't know that I would class the ultrasound as rude.