The accepted answer works well. Just for completeness (and since I like it's shortness), I wanted to share how to to it with compass (SCSS/SASS):
body{
$colorStart: rgba(0,0,0,0);
$colorEnd: rgba(0,0,0,0.8);
@include background-image(linear-gradient(to bottom, $colorStart, $colorEnd), url("bg.jpg"));
}