Problem in Rendering: Sequencer: Black border creeping upward

Hi, folks

I have a problem that I have not been able to figure out, and it is this black border that is creeping upward when I take images and render them out in the AVI Jpeg Format. What I am trying to do is re-use rendered images over and over again, to add more and more background layering. But each time I render out the AVI Jpeg file from the sequences of images, this black border at the bottom gets WIDER and WIDER…cropping the image more and more. I tried “crop” and “border” to no avail and I played around with the presets (I’m rendering at 1920 x 1080)

To give you an example, I start out with this original Targa, which took two minutes to render (due to memory considerations):

The first image is just the airship. Next, is the image of the airship, rendered as a plane image (billboard) with the tree in the foreground, and the third image is the same image fed through the sequencer and rendered again, just to see if the border increases. Say I wanted to render an image 10 times, adding something new each time (to reduce number of verts and polygons in a scene) How would I keep this black border at the bottom from creeping all the way up and chopping out my image?

Any help would be greatly appreciated. I’m using Blender 2.5