Add two more layers to the background movie clip one called
“mask” and one called “masked”. In the layer called masked create a
movie clip called “masked background” containing the colour picture of the
background. In my example this is the colour picture of the taxi. Make sure that
this layer is aligned exactly with the black and white version of the picture
using the align buttons. Lock this layer as well so that you don’t move the
picture.
You should now have the two pictures on the stage: the
colour one sitting on top of the black and white version. Now it is time to
create a shape that will be “dragged” across the two pictures: the mask.