you can do it with the clone tool :
1/create a new solid
2/double clic on it
3/select the clone tool
4/in the paint panel select your footage in the source menu
5/clic the incrust button and adjust the opacity of you footage (you see it when you mouse is on the solid window)
6/now you can reveal you footage with the clone tool
this paint solid is now the new version of you revealed footage so you can switch of the original
voil