Why doesn't this background image look correct?

Head out to http://brucegust.com/partners/header.php

My background image looks fine in IE 8 and Safari, but it's shifting to the right in IE 11 and I don't know why.

While you can see my entire stylesheet at http://www.brucegust.com/partners/stylesheet.css, the portion that applies directly to my background image is:

img.bg {
  /* Set rules to fill background */
  min-height: 100%;
  min-width: 1620px;
      
  /* Set up proportionate scaling */
  width: 100%;
  height: auto;
      
  /* Set up positioning */
  position: fixed;
  top: 0;
  left: 0;
}

@media screen and (max-width: 1620px) { /* Specific to this particular image */
  img.bg {
    left: 50%;
    margin-left: -553px;   /* 50% */
  }
}

#header_top {
position:relative;
width:976px;
height:135px;
background-image:url(images/header_top.png);
background-repeat:no-repeat;
margin:auto;
margin-top:10px;
}

What do I need to improve in order to ensure a full screen background image in IE11?

Thanks!