Adding background image to div using CSS

It is happening because .header-shadow is empty.

Add height to it:

.header-shadow{
    background-image: url('../images/header-shade.jpg');
    background-color: red;
    height: 50px;
}

Fiddle here.

Specify a height and a width:

.header-shadow{
    background-image: url('../images/header-shade.jpg');
    height: 10px;
    width: 10px;
}

Add height & width properties to your .css file.

You need to add a width and a height of the background image for it to display properly.

For instance,

.header-shadow{
    background-image: url('../images/header-shade.jpg');
    width: XXpx;
    height: XXpx;
}

As you mentioned that you are using it as a shadow, you can remove the width and add a background-repeat (either vertically or horizontally if required).

For instance,

.header-shadow{
    background-image: url('../images/header-shade.jpg');
    background-repeat: repeat-y; /* for vertical repeat */
    background-repeat: repeat-x; /* for horizontal repeat */
    height: XXpx;
}

PS: XX is a dummy value. You need to replace it with your actual values of your image.