[WP Ecommerce] Can't Manage Product Thumbnails

I’m using WP Ecommerce and I’m trying to add a thumbnail to a product, but for some reason, when I click on the Manage Product Images link and choose an image, I don’t get the option to add the image as a thumbnail. I only get the option to delete it.

I’ve seen one other person mention this on the WP Ecommerce forums, but he never got a response, so I’ll ask it here.

I’m just wondering if anyone has ever seen this issue before and might know how to fix it.

Thanks