Billy,
Pretty easy...
Use the
VB Code Enhanced Mode in your post, then add the URL Hypertext Link code. Next, insert the IMG code where the "clickable text" normally goes (in between the opening and closing tags of the Hypertext code.
The address for the IMG code should be that of your thumnail image, and the address in the Hypertext URL code would be that of the web site you want the thumbnail to link to.
The structure is something like this...
URL CODE - WEB ADDRESS - IMG CODE - THUMBNAIL ADDRESS - END IMG CODE - END URL CODE
I can't type the actual code in 'cause it will run it and you'll see what I have in my above post, not the actual code.
There are instructions here in RCU at
http://www.rcuniverse.com/misc.php?action=bbcode
Good luck,
Tom