Hi,
I'm not sure what is your problem/question.
I don't think that adding the main image in the cart/email without thumbnail is a good idea.
This is the main image on your link:
www.artnow.it/3/images/com_hikashop/uplo...rtoline_28642474.jpg
It's 1000px wide. If you would display it without thumbnail on the cart/email, the layout would be a mess.
Do you mean that you want to have the image thumbnail a bit bigger in the cart ?
In that case, you can simply configure the size you want with the options "Thumbnail width" and "Thumbnail height" in the images section of the configuration.
For the emails, there is no option. You would have to edit the emails via the menu System>Emails and change the code: array(50, 50) in the "preload" area of each one.