Concept by collage is a really fun exercise! The idea is to create concept art only using existing images (Photographs, Art, Textures, etc.)
I started the below concept with an idea and a simple sketch. Then I scoured the internet looking for the right images. Much of the pieces may look like they were already together but if you look closer you can see some of the seems. After it is pieced together I used "Curves", "Hue and Saturation", simple "Mulitply" washes, a painted in Rim Light, and a good old fashioned "Lens Flare" in Photoshop to tie it all together.
If this were for a real project I would spend time painting over the entire image to make it my own. Project completion time - 5 hrs