Change product image size.

Hi,

I’ve been trying to change the product image sizes from square to rectangular.
I searched on the internet, but nothing worked for me so far. Stylesheets, functions.php
Please can someone direct me in the right direction?

It involves the catalog:
http://www.johanjdenooijer.nl/winkel/

And the product page:
http://www.johanjdenooijer.nl/winkel/filippa-de-verkrachting/

Thank you.
Chantal