Here is a composite I tried to realistically integrate the photograph of a person into CG world. Please let me know how can I improve this. It was rendered using Cycles and composited in Photoshop.
You could improve lights and shadows. Those from the environment should correspond to the ones on the girl. Her silhouette needs to be lighted from above and the back (backlight).
You can try to put the model, simplified human figure in the environment, to use this as a reference for your work. Just to see, how the light and shadow on the figure looks like.
Good luck with the project, should be interesting.