Add a class to your external links as

give background property to external class as background: Url(external.gif) left center;

This will add image to the left of your external link.

A[rel="external"], a.external {
  white-space: Nowrap;
  padding-right: 15px;
  background: Url(/images/external.gif)
no-repeat 100%
50%;
  zoom: 1;
}

<a href="http://www.externallink.com"
rel="external" target="_blank">link text</a>