I have a family photo but three people are missing. Is it possible to add them in with photos of them?
Yes. You can supply reference photos of the other people you want to add if you also provide detailed instructions. Keep in mind how they can be added and how natural it will end up looking depend on the quality and orientation/angles of the other photo you are trying to add.
Yes, things and subjects from multiple photos can be merged together into one painting if the orientations are conducive to doing so. To see examples of this being done you can check out our gallery at https://www.instapainting.com/gallery/merge-photos