CSS - Set Div Width 100% and Resize Keeping Aspect Ratio

Question

I have a div with a background image that will overlay part of the header slideshow. I want the width of the div to always be 100% of the window size, even when the user re-size

Answer 1

This somewhat distorts the image, but it might be what you are looking for:

#bg_img {
    background: url('http://rndimg.com/ImageStore/OilPaintingBlue/999x400_OilPaintingBlue_19aa91c1b6e142f288fe69eb2a160a2b.jpg') no-repeat;
min-width:100%;
min-height:100%;
background-size:cover;
}