Correct, because then the browser still has to wait to receive the original image header itself in order to calculate the "missing" dimension, and you're right back to the original problem.
Supplying only one dimension works fine if your only concern is to grow/shrink the image while maintaining the original aspect ratio, but it unfortunately doesn't help the problem of some browsers (*cough*Netscape*cough*) not wanting to display the rest of the page until it can figure out the image size(s).
Thanks for the help and information . . .