Sometimes you see a photo taken by a friend that you would love to paint. Just ask them if they would mind your use of the photo for reference. Be sure they know you are not painting it for them, but for your own collection, or for sale. If the photo was taken by a professional, get a signature of use document signed. Keep one for your files and have the professional keep one in their files. I fell in love with a photo taken by Rose Margaret Braiden of calla lilies. She was very happy to allow me to use it as the reference for this painting.