grainy product picture

Hi Dear,
Can you please tell me why a product picture is grainy?
This is the subsite: http://www.pixshopping.com/4m4progettazionistore/
Attached a screenshot.
I did also a test as you can see.
The picture seems to be good and is like all the others.
I give you support access in case you wanna check it.
thanks,
Paddy