var counter = 0;
var header = [];

header[counter++] = ["header1.jpg"];
header[counter++] = ["header2.jpg"];

var whichHeader = Math.round(Math.random()*(header.length-1));

document.write('<div align="center">');
    document.write('<img src="/images/' + header[whichHeader] + '" width="740" height="112" alt="ASHT - American Society" usemap="#Map2" />');
    //document.write('<img src="/images/header.gif" width="740" height="112" alt="ASHT - American Society" usemap="#Map2" />');
document.write('</div>');