Logo low image resolution on smartphones

Hello,
on website www.yerbasantashop.it I've uploaded a logo that displays great on a desktop browser but on smartphones its resolution gets worse. Why?
On the website there is a template bought on the nopCommerce marketplace. I've tried also to set the default nop template with the same logo and other different logos but the result is the same: on smartphones the logo resolution is very low.

Thanks

What version of nopCommerce ?
What logo ?
The images on the site looks ok to me on Mobile iphone
What theme are you using ?
If you still have issues then if you purchased then best contact the developer / service provider for the theme to see if they can help