The simplest way to do this would be to add:

margin-right: auto; 
margin-left: auto;

to the .mainimage class in template_style.css, but this will center the main image as well.

If you would like to center the thumbs but not the main image, duplicate .mainimage as .allthumbs (for instance) and add the above code, then replace the

<div class="mainimage">

in template_archives.php with

<div class="allthumbs">