Removing custom headers from posts

Hi,
My theme has a nice large header image which is fantastic for desktops but looks a bit too much on mobiles. I tried different ways of making them more like a banner (for mobiles only) adding margins from top and bottom etc. but failed.
I use different custom header images for each post under certain categories which makes css quite complex.
Because I failed to manipulate the height of my images I am thinking now about removing them on mobiles.
I would really appreciate your help!
My site – http://fanscorners.com/