That's a little beyond the scope of support, but there is an existing post that may help a little.

http://www.markettheme.com/for…..thumbnails

I believe the line numbers are off, because it's referring to the "Market Product Manager Plugin", which has since been integrated into the functions.php file of the theme.  But if you were really dedicated, you could download a copy of the plugin from the download center to reference.  Then search the functions page code for the code.