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