So I put this "fixed position contact tab" on the side of my website and I cant get the image transparency to work. Any ideas? What am I doing wrong?

Here is the CSS code:

<style type="text/css" media="screen">

#contact_tab{
  left: 1px;  /* change this to left: 1px; to put it on the left of the screen */
  top: 30%;
  height: 125px;
  width: 50px;
  margin: 0;
  padding: 0;
  position: fixed;
  border: 3px solid black;
  filter:alpha(opacity=55);
        opacity:0.55    
  z-index: 5000;
  background-image: url(../ContactTab.gif);
  background-repeat: no-repeat;
}

#contact_tab a{display:block;width:100%;height:100%;}
</style>