How to change size of the thumbnails appearing in the product details page

[wiixplay]

Hi All, I've tried to create a product and added a few pictures to it. When I go to the products page, it shows the main picture at the right size (i've changed it to 500x500 in the THICKBOX of the preference/images from the BO). The only thing is that the small thumbnail pictures under it don't show up correctly. The only way I found was to set the MEDIUM size to 50x50 but it changes all the pictures on my entire site....is there a way of changing only the thumbnail size of the product image with affecting anything else in my site?

 

Here's an example of my issue:

 

http://www.xkase.com/fr/%C3%A9tuis-coques-housses-apple-iphone-4-4s/10-coque-en-aluminium-iphone-4-4s.html

 

Thanks,

 

Daniel