Works:
<div id="mapOhio"> <img id="myImage" src="<?php bloginfo('template_url'); ?>/images/ohioMap.png" width="253" height="285" alt="Image of a map of Ohio." usemap="#image_map" border="0"/> </div>

<!--[if lte IE 8]>
  <script>
    $("#myImage").attr("src", "/wp-content/themes/TRTheme/images/Chester-4317.jpg");
  </script>
<![endif]-->


Doesn't work:
<div id="mapOhio"> <img id="myImage" src="<?php bloginfo('template_url'); ?>/images/ohioMap.png" width="253" height="285" alt="Image of a map of Ohio." usemap="#image_map" border="0"/> </div>

<!--[if lte IE 8]>
  <script>
$(document).ready(function(){
    $("#myImage").attr("src", "/wp-content/themes/TRTheme/images/Chester-4317.jpg");
}
  </script>
<![endif]-->