At some point you stop redesigning the composition of the painting and you have to start making marks on the big piece of paper - just DO it!
Here I am placing the red cactus flowers in the foreground. Because of the camera lighting, you cannot see that I am standing in a dark room. The paper is taped to the door and behind me is a photo projector, sitting on an ironing board.
I have found this to be the best way to combine several photos into one composition. You can move the projector forward or back, up or down, until the size and location of each photo is correct and melds into what you have already drawn.
Texas Hill Country Art