thickbox function in my site is not working – wp ecommerce plugin issue?

The problem is that the thicbox function is not working on my product images in my site.

I don’t think I have done anything to disable this function. (People are clicking on product images and instead of getting the thickbox javascript function, a new window opens in the browser with the single image): https://micamiseta.com/tienda/honduras/
I am using WordPress 2.8.6 and WP Shopping Cart plugin Version 3.7.5.2. Could it be I haven’t updated to the latest versions?

Any idea where I should look to fix this?? I need to get this working asap. Thanks!