Thumbnails Problem

Hi, I am having a problem with some of the product thumbnails on my site. The problem arises every so often, and is associated with particular blogs on the network, sometimes it rights itself after some time, and it is not happening when I switch to other themes, so it is particular to the Network theme.

You should see what I mean on this link:
http://fondelifair.com/marketplace/categories/baby/
and here is a link to one of the problematic products:
http://decorativeartwork.craftblog.eu/store/products/bunny-with-balloon/

Thank you!