<script>
function reload () {
var blabla = document.getElementById('img');
var img = new Image;
img.src = '../kcaptcha/index.php?session_name=<?=session_id()?>';
blabla.src = img.src;
}
</script>
<!-- Это капча --!>
<img id="img" src="blablabla/yousuck.jpg" />
<input type="button" onclick="reload()" value="RELOAD"/>