Not a a native english speaker so there\'s probably a better way to shape the question...anyway:
What I want to create is similar to the header here: http://thegreatdisc
You just need to pass the height to width ratio to the element. For an image 1400x600;
1400:600 = 98:42
span( style="padding-bottom:42%;
width:98%;
background:url('/images/img1.jpg');
background-size:contain;
display:inline-block;")
would display the same as
img(src="/images/img.jpg" style="width:98%;")