Background images: how to fill whole div if image is small and vice versa

Resize the image to fit the div size.
With CSS3 you can do this:

/* with CSS 3 */
#yourdiv {  
    background: url('bgimage.jpg') no-repeat;
    background-size: 100%;
}

How Do you Stretch a Background Image in a Web Page:

About opacity

#yourdiv {
    opacity: 0.4;
    filter: alpha(opacity=40); /* For IE8 and earlier */
}

Or look at CSS Image Opacity / Transparency

To automatically enlarge the image and cover the entire div section without leaving any part of it unfilled, use:

background-size: cover;

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