Here's an example of a large background image, solid black background, and text in a semi-opaque black box.

The CSS code looks like this:

body {
     padding: 0;
     margin: 50px;
     background: #000000 url(02484_passerellesurlerhone_1280x1024.jpg) no-repeat center top;
     width: 100%;
}
p {
     color: #fff;
     width: 450px;
     background-color: #000;
     opacity: 0.65;
     padding: 10px;
}