Can I set background image and opacity in the same property?

Two methods:

1. Convert to PNG and make the original image 0.2 opacity
2. (Better method) have a <div> that is position: absolute; before #main and the same height as #main, then apply the background-image and opacity: 0.2; filter: alpha(opacity=20);.

#main {
    position: relative;
}
#main:after {
    content : "";
    display: block;
    position: absolute;
    top: 0;
    left: 0;
    background-image: url(/wp-content/uploads/2010/11/tandem.jpg); 
    width: 100%;
    height: 100%;
    opacity : 0.2;
    z-index: -1;
}