This seems like such an obvious feature that should be in wordpress. I'm not sure how the devs missed it.
When creating a new post you can upload images which are then put in a gallery. Unfortunately you cannot put images previously uploaded in that gallery. Why? It makes no sense. It makes it so that images I uploaded yesterday can't be used in a gallery I want to post today.
Put simply: Make a way for the user to be able to select which images they want in which galleries. It can easily be done with a button and a few check boxes.