The reason this final image looks so convincingly real is that the shadows all fall in the same direction. When choosing source images for a composite such as this one, it's absolutely essential the shadows match in foreground and background. The technique I used here was to first remove the background of the original photo with the kids, using the Bezier pen tool in Adobe Photoshop to precisely isolate the kids and the rocks, then save that path as a selection and make it its own layer, and from there apply a layer mask to it, so I could fine-tune the edges of the rocks and the kids—making them smooth in some places like the hair and sharp in others like the rocks—which then allowed them to be placed seamlessly on any background, which in this case was the Manhattan skyline as seen from the Empire State Building. After combining the two photos, the final step was to convert the image to black and white to achieve maximum tonal continuity between foreground and background.