is option selects the basic vertical position of the overlay image
with respect to the underlying image, in syntax reminiscent of HTML.
<b>top</b> means flush top, <b>middle</b> means centered, and <b>bottom</b>
means flush bottom.

<p>The <b>-yoff</b> option modifies this position.

<b>above</b> means just out of frame to the top -- the bottom edge of
the overlay is flush with the top edge of the underlying image.  
<b>below</b> means just out of frame to the bottom.  These alignments
are useful only if yo