Header Image consistency across all devices

Hi all, I want to have the very same header image across all devices, desktop to mobile, at the moment i have an image that suits perfect for desktop but crops the photo rather than reduces the ratio when on a mobile device. Is there a way to add some CSS code to change this or an internal option/plugin so that the image i look at on my desktop is the very same image that's on my phone? Any help is appreciated, Thanks