First off, I promise I will post ultrasound pictures at some point. I'm just lazy and haven't scanned them in, yet. I promise, though, it looks like a shadowy, more-or-less baby-shaped entity. :)
Anyhow, we've shown Miles the pictures, and he now understands that it's a picture of a baby. The first time I showed it to him, I said, "What's that?" and he said, ". . .Whas dat?" Once I pointed out the nose, belly, and hand, he kinda got it. Now, when I show him the picture, he says, "Baby!"
Once that was established, I told him, "That's the baby in Mommy's belly!" To which he responded, "No. Silly!" And giggled. Because the idea of a baby in Mommy's belly was ridiculous. We repeated that particular conversation for over a week.
Yesterday, he finally accepted that the baby was in Mommy's belly, so I moved on to the next topic. "You're going to be a big brother!" "Big Brother?!?" he said, proudly, sitting up straight and tall. "Yes!" I said, excited, "You'll be a big brother and help me with the baby. You'll help me get bottles, and throw away diapers, and get the pacifier. You'll be a great big brother!" He looked at me skeptically, then grinned and said, disbelieving, "Noooo."
Later, I asked him if he wanted a baby brother or a baby sister. He thought for a minute, considering the options, then replied, "A baby." I tried rephrasing the question, asking if he wanted a girl baby or a boy baby. He again replied, "A baby." :)
Even later, we were reading a book, and he was being silly. He put the book on his head and said, "Is on my head!" Then, repeated the game with, "Is on my eyes! Is on my nose! Is on my tummy! Is on my baby!" I laughed and explained that only Mommy had a baby in her belly. He raised an eyebrow, Spock-like, and said, "Nooo."
In the end, I'm not sure if he really gets it at all, but it certainly is making for some cute conversations. :)