Hi there!!!
Well, as you say, you use IIS server to run your site.
This mambot need the GD library to make the thumbnails of the original images.
If i'm remember right, there are a "server module" for IIS that is installed and used from the IIS server to make use of this library.
As you can see, if you go to the link
http://www.steveonline.ca/mambots/content/plugin_jw_sig/showthumb.php?img=family/LifeJacket/dscn0935.gif&width=100&height=100&quality=50
will see that browser respond
<b>Fatal error</b>: Call to undefined function ImageCreate() in
<b>D:\InetPub\SteveOnline\mambots\content\plugin_jw_sig\showthumb.php</b> on line <b>45</b>
that means there are no installed the "GD library" module to your IIS server.
Better contact your server adminstrator to tell him to install this IIS server module!