skin/reboot/css/reboot.css
Code:
.thumb_div_medium { position: relative; margin: 0 auto; width: 127px; height: 127px; }
If you don't want the thumbnails to be confined to a specific size, in skin/reboot/customer/main/products_t.tpl
Replace...
Code:
<a href="{$url}" class="unify thumb_size_medium">
With...