Could someone tell me how to create square product image thumbnails? Or ViArt can create only rectangle thumbnails?

I believe that the thumbnails are based on the original image they are being created from so if that image is not square, the thumbnail won't be either. If they automatically made them square, the image would not be proportional. (They would end up stretched or scrunched vertically or horizontally.) But I agree that it would be cool if there was an option to have images resized proportionally and then add white space (evenly) to make them square. Not being a software person, I don't know how doable that is though.

There is a free program called Phatch which is cross platform for batch processing pictures with many different processes.
http://photobatch.stani.be/
 
You need to create an "Action List" of commands to perform and theres a tutorial at
http://photobatch.wikidot.com/