Possible? Downsampling Header for Retina

Is there any way to downsample the header image for retina displays in ATO or in the code? I'm not experienced with PHP coding so I would need detailed guidance if I were hardcode, and I'm not looking to for a "retina recognition" option swtich, because I don't care whether it takes two seconds longer to load as it's only one image and not that big a deal Anyone with experience on this?