Header image pixelated / custom header alt is pixelated

Hi there - I am trying to make the main theme header image area larger. Right now it cuts off the image in both boxed and fluid even though i made the image exactly 1920x1080. Is there a way to make it fixed 1920x1080? I tried to make my own custom header (by hiding the theme header and just inserting a "header" module" but then it is pixelated when I publish it. Any advice would be helpful. Thanks Brindl