Two things jump out at me right away in your image. The first is that the lighting for the model and the background are completely different. This creates an incongruity between the two. The second thing is the perspective of the model to the scene. She would have to have very tall legs to be that high above the beach. Or, she would have to be standing on something we can't see. Either way she looks out of place to the background.
Here is a "fake" photograph that is comprised of four photographs; the model, the bike, the arboredum, and the reflection of the model on the gas tank of the bike. I didn't original shoot these elements with this idea in mind but playing around with the various elements this picture evolved. All were shot in indirect light.
In the second photograph the model and background were taken years apart, one with digital and the other with film. The angle of the sun was the same for both which helps to make it believable. Since the background isn't important and to separate the model from it I blurred it. The sun is harsh coming in from the model's rightside. I wish I knew about using a scrim then. In fact, the eyes have been opened by bringing them in from another picture. One good thing was I got a lot of reflective light from a white wall just to the right of the model as a fill light. That was dumb luck at the time. In these forum's I have learned how JT and others use this trick all the time.
Both of these images above were original created before I discovered this site and now I know at least more about what I need to look for in the composition of my pictures.
I don't claim to be an expert in "faking" pictures, so if anyone would like to take a shot at them please feel free to make comments.