Animate it...
In the preview screen, use the POSITION effect to drag the overlay image off-screen to the starting location. Set a keyframe by clicking the small green + sign.
Then move the scrubber (red line) in the effects window to the position where the action is to end. Set another keyframe.
The graph would look like this for a one second clip if the image drops from the top to the middle of the frame within a half-second, then holds still for remaining half.
In this example the overlay image has been scaled down to .30. But that may not be necessary in your case.
For animation instructions see http://help.nchsoftware.com/help/en/videopad/win/effects_animating.html