"Real" thumbnails, not resized fullsize images

Someone just pointed out why the images on our "Browse Images" page are so slow to load (see http://mith.umd.edu/larsen/items/browse?recent=false)--it appears that rather than displaying the 200x200 versions of the images in archive/thumbnails, the site is drawing the fullsize images (either from archive/files or archive/fullsize) and shrinking them to fit the thumbnail dimensions I entered in the admin interface.
I'd like the browse page to just display the smaller (200x200) images from archives/thumbnails if possible so that the loading isn't so slow. Currently, I'm using this code to put the "thumbnails" to the left of each of the item descriptions:

<?php echo display_files_for_item(array('imageSize'=>'thumbnail')); ?>

I thought that would point to archives/thumbnails, but as I described, it is not. Any advice on how to get that to happen? Thank you!

I think it is because you have echoed the file for display rather than the thumbnail.

Have you tried using a different function:

<?php if (item_has_thumbnail()): ?>

<div class="item-img">--or whatever class you've designated

<?php echo link_to_item(item_thumbnail()); ?>