function roll(img_name, img_src)
   {
   document[img_name].src = img_src;
   }
<!--
document.writeln('<a href="index.html" ');
document.writeln('onmouseover="roll(\'sub_but\', \'images/header_hl.png\')" ');
document.writeln('onmouseout="roll(\'sub_but\', \'images/header.png\')">');
document.writeln('');
document.writeln('<img src="images/header.png" width="236" height="65" border="0" ');
document.writeln('alt="Ted Davis | performer of multiple tasks" name="sub_but">');
document.writeln('');
document.writeln('</a>');
//-->