I studied graphic design in college, and in my first computer graphics class, one of the big projects was to create several photo montages and make them look realistic. We had to take something that obviously did not belong in a certain background and place it there in such a way that made it look natural.
This took many hours of hard work, because I was a beginner at using photo editing software. I put an Eskimo in the desert, a tiger in a field of flowers, and an elephant in someone's front yard.
It involved a lot of copying different pieces of sand, leaves, and flowers and pasting them around the feet of the creatures to make them blend into their new surroundings. I also had to cut the original background out from around the creatures so that their edges didn't appear jagged.