you can use the matrox realtime plug-in to apply a chroma key to make certain areas of a foreground video clip transparent based on a color in that image, so that an underlying video clip can show through.

    in the following example of a chroma key effect, our foreground image is a video clip of a woman sitting in front of a green backdrop, and our underlying image is a video clip of a sand dune:

   

    by chroma keying on the particular shade of green in the backdrop of the foreground image, the backdrop area becomes transparent and the corresponding area of the underlying image shows through in the composite image as follows:

   

    you must apply the matrox chroma key effect to the clip that contains the colors on which you want to key (that is, your foreground image on a superimpose track). both the foreground and underlying images must be video clips. if you want to use a graphics clip, you can export that clip from the timeline to a single-frame .avi file, then replace the graphics clip on the timeline with your exported file.