To movie the picture around inside a frame you will need to:
1) Find the clip you that has the Green Screen overlay on it.
2) On top of this clip you will find the Green Screen clip sitting on top with a green boarder around it.
3) Click the Green Screen clip to highlight it.
4) Once this clip is selected look to the top left hand side of the preview window. You should see a small menu bar with two options in it, "Full", or, "Cropped".
5) Click on, "Cropped", and the picture should appear in the clip you put it over. Use the four corner re-sizers on the corners of the picture to movie it to where you want on the clip.
Hope this helps but it doesn't drop a comment about what you don't understand or what didn't work and i will do my best to help.