The reason I wanted to use an image from the gallery is so the users won’t be confused as to why they need to add the same image twice if they want it to be the featured image and be in the gallery. They’ll all do it differently. One will add it to both places, one will add it to featured image and not the gallery, and another will add it to the gallery and will have no featured image.
I had thought of just using the first image in the gallery. That would be fairly easy, but in some cases, the gallery will need to have a distinct order to it, yet the first image may not be the most representative.
