http://www.w3schools.com/css/css_background.asp
http://www.w3schools.com/css/css_image_transparency.asp

example:

codice:
 
CSS

body
{ 
background-image: url('fiore.jpg');
background-repeat: no-repeat;
background-attachment: fixed;
background-color:green;
}

#table1
  {
  width: 400px;
  height: 180px;
  margin: 30px 50px;
  background-color: #ffffff;
  border: 1px solid black;
  /* for IE */
  filter:alpha(opacity=60);
  /* CSS3 standard */
  opacity:0.6;
  }