Add a person to a movie:

Download the movie still here and the person to be added here.

Open photo of person to be added
Crop so only the person is showing. Make sure to not cut off any of the person.

Open movie still
Find image size in pixels

Reduce the height of the image to be added to match the height of the movie still. Be sure Constrain Proportions is turned on.

Under Select, choose Select all on the photo to be added. Paste the image into the movie still. Apply a mask the new layer.

Hide the background layer. Using a hard-edged brush, mask away everything but the person to be added.