In my category item view after 8 different products are shown the 9th and 10th thumbnail images change sizes from 100 to 141. Here is the link: americanvending.com.previewdns.com/index.php?option=com_k2&view=itemlist&task=category&id=30:video-games&Itemid=59

If you inspect your page and look at the image sizes with firebug in the span class itemImage
you find the first image is 125 wide/autoheight
the second image is the same
the third image is showing 100 wide auto
There may be another way to solve this problem, but the only way i have found to do it, is to create all the images by cropping them to a standard dimention and then when the resize happens, and the aspect ratio is preserved they will all be the same width and height.