How to create a retina header in old theme

Normal pictures on my websites i have uploaded in double size. They are cropped automatically to the given width and heigt in html (ex. Original file 300×150 to html size 150×75).
At this moment i am also trying to create a retina header. The size of the header is 874×94 and i have created a header of 1748×188. If i upload this file the header will not be cropped.
How can i change that in my css file?

I found this 2 rules:
div.Header{margin:0 auto;position:relative;z-index:0;width:874px;height:101px;}
div.Header-jpeg{position:absolute;z-index:-1;top:0;left:0;width:874px;height:94px;background-image:url(‘images/Header.png’);background-repeat:no-repeat;background-position:center center;}