It is true that i's because it's a link, however, I'm not using the thumnail.tpl, I'm calling the images like this in the products.tpl :
<div align="center"><a href=product.php?productid={$products[product].productid}&cat={$cat}&page={$navigation_page}>[img]{$ImagesDir}/image/{$products[product].imagecode}[/img]
{ $lng.lbl_see_details }</a></div>
Any other ideas ?