I tried merging some of my other photos within the previous shots, some were more successful than others. I found that to achieve the best result the location image ought to match the perspective of that of the hole it was being placed within.
I also found that having elements of the image coming out and merging with the outside/real life image made it even more effective/believable.
I experimented with some illustration work as opposed to photography. I do not own the illustration work - I used them as examples to test how it looked.