[theOZer]

6piston,
Concerning older versions of IE. Trying to cater your forum to work in older IE versions is honorable but… why? If someone has IE7 upgrade or IE8 upgrade (dropped today!) or, worse yet, IE6 upgrade "fear," smolly. Upgrade. Much to my amazement and joy was the discovery that the latest version of Mozilla’s FireFox seems to be THE browser to use when viewing a vB forum. So, put a ‘works best with…’ line hidden somewhere in your footer using a really really small font (visualize a cute smiling smilie here).
No. I apologize and after rereading see my suggestion as misleading about performance. But, have you tested using width & height? To actually reduce your thumbnails’ size (AdminCP > vBulletin Options > Message Attachment Options > Thumbnail Size) might improve display delay but its messy and not recommended by me. Always first test changes like this in your testvb.
My initial post here about using width and height was for someone who just wanted to tidy up, display smaller thumbnails in a list.
Nice web site and utilization of this TT mod. Remember, any hookname that “knows” a thread’s thread.firstpostid can be used to grab 1st post attachment info, too. For example, I found one in tags.php, hookname: tags_list_query_data (see pg 12, post 169). How you’ve shaped up your forum shows me you should be able to take this FYI tidbit and run with it. Good luck.
Regards, theOzer
P.S.
Curiously, IE8 was officially released as I (pre)typed my reply diatribe here. IE8 seems to "handle" using HighSlide in Image Resizer (which I use with NO image resizing, just HighSlide). Hmmm...