Header Image is perfect on desktop but blurry on mobile

Hi there, no matter what i do the header image is perfect on desktop but unacceptable on mobile devices, since i am a photographer this is really frustrating. I have tried everything i could, small images, medium, huge, different dpi's etc, and the result is always the same. The only way it was ok on mobile was when i cropped the photo i made it portrait, but then on desktop looks nothing like i wanted. Thank you in advance for any ideas you may have!