I would question the genealogical relevance of a baby scan. It is essentially just a photo, just as that photo of your son's third birthday, or the first time he rode a bike. These photos are not added as facts, because they are not of much genealogical relevance. So the most logical place to put it is just as an image or media.
Certainly this baby scan is part of your family history. If you decide to author a book or publication about your recent family history then this scan could be an interesting image to include.
As for adding this to your tree, presumably a young person's profile is private except for any close relatives you might grant access to living person data. There will be very few people who see this, so it is really up to you how you present the image.
And finally, just a note of privacy. An ultrasound scan is harmless enough, but it is medical data. Of course, it is not uncommon for people to post their baby's ultrasound on social media these days. But your son is nearly of age, and could reasonably object to having such an image posted in some public way. In any case, get permission of the subject before posting any images of living persons to your family tree.