My wife and I sponsored our son's wedding a few years ago. It was held overseas, and we paid for everything. We paid for a photography session that resulted in some wonderful high-quality wedding photos. One was a portrait of our son and his bride.
Just recently I brought the large portrait photo and frame back to their home in the States so the still-happy couple could hang it somewhere in their home. My daughter-in-law heard me reassembling the frame and asked what I was doing.
When I told her, she said, "Oh, don't bother. That (hanging the photo in their home) is not our style."
I was flabbergasted.
After paying for the wedding, reception and photo sessions, I would have thought they would hang the photo in a prominent place in their new home (for which we also contributed the down payment). Instead, I was told, "We will probably put it in the spare bedroom." In short, no one would ever see it.
"Not our style" was an unwelcome thanks for all we had done for these two, and it seems as if they do not know how to express gratitude, or perhaps are merely too selfish to understand what they were given.
Providing a down payment on a home for your son and his wife does not then obligate the couple to furnish it according to your taste. It does, however, obligate them to express their gratitude to you. It doesn't sound as if they have.
An appropriate response from you is not to insist that they place this photo in a prominent place, but to reconsider your generosity to them in the future.