[Plugin: WP e-Commerce] how to get thumbnail image url for particular product ?

Images must be thumbnail but when I click , it should open full image in thickbox . But it is showing full image on both . It is using this function ‘wpsc_the_product_image()’ to get src of the full image , is there any other function so that I can get thumbnail src ??

https://www.ads-software.com/extend/plugins/wp-e-commerce/