Once you drag an element into the photo you must determine where you want to place it and what the lighting conditions are in that area. You may need to flip it so it matches the direction of the light. You may also want to blur the image to mimic depth of field. The better you understand light the more realistic your images will look.
We photograph most elements in a shaded conditions before it is extracted from the background. We also provide them fully saturated with natural color. In this manner, you have full latitude to desaturate color, change brightness levels and add highlights where you feel necessary