function autoexec()
{

    Event.observe($('image100'), 'click', changeImgPath100, false);

}
function changeImgPath100(e)
{
    var org_action = $('myform').action;
    $('myform').action = 'preview100.php';
    $('myform').target = 'preview_iframe100';
    $('myform').submit();
    $('myform').action = org_action;
    $('myform').target = '';
}

Event.observe(window, "load", autoexec);