Mobile header photos resize code

[JaneNZ]

Hi there I am keen to get some code added to my CSS to help resize my short header images to sit better on mobile devices, currently they are cropping off or I have to increase my short header size to not be short which takes up way to much space on the desktop. I do understand that the format for desktop and mobile are different, some images seem to work better than others. Website I am working on is http://www.theelementsspa.co.nz 

Thanks in advance.

[wireandwings]

I need this as well. My hader image is a banner with the company name. Making it a square for viewing n the computer is not an option, but I can design a square one to be used only on mobile sites. Right now my banner image shows up so tiny it can't be read!