Try this (what I added is in bold):
Basically, all the table code looked ok, so then I looked into the graphic itself (i substituted a graphic I had for testing). adding align="bottom" didn't work. I tried align="middle" for the heck of it and it worked. So try that and it should fix it, as long as you don't put any text in the top table, then you would have to use CSS, which older browsers don't support.
Tested on IE 6.0 and Opera 6.01