Hi again
Here is a mockup of what I think you may be after... Main Track image is your "Space" image (Actually M42 in Orion) The overlay image is a .gif image with a transparent background and "open" eyes.
I have used the Zoom effect to set the "Space" image large enough to scroll with the AR restrained to 16:9 and the rectangle set at the extreme left of the image. The red cursor line in the effects window is set at the start of the clip (left end) The 12th. tab (small green cross with lines) on the effects window is then clicked to set the 1st keyframe.......

Next, the red line cursor in the effects window is slid to the end of the clip. (Right end) The zoom frame is then moved to the right side of the image keeping it at the same height and a second keyframe is set......... (reversing the positions of the zoom frame will scroll the clip in the oposite direction)

The zoom rectange will automatically show full frame behind the overlay face and will be seen through the "eyes" as well as they are transparent. On playing the clip the "Space image" in the background will slowly (depending on the duration of the clip) move from right to left behind the overlay face.'''' (Note what I said about the position of the zoom rectangle for each keyframe) Here's the the type of effect seen for this at the start-mid point and end of the clip. In this case the duration of the images was 10 seconds, The amount of zoom and the duration of the images will alter the speed of the effect.



Hope this helps and is something like what you are after.
Nat