function getrandomString() {
	var chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghiklmnopqrstuvwxyz";
	var string_length = 8;
	var randomstring = '';
	for (var i=0; i<string_length; i++) {
		var rnum = Math.floor(Math.random() * chars.length);
		randomstring += chars.substring(rnum,rnum+1);
	}
	return randomstring;
}

rndString = getrandomString();

document.form_reserve.codetext.value = rndString;

captchaURL = "<img src=\"CaptchaImages.php?width=100&height=20&characters=8&code=" + rndString +"\" />";

document.getElementById('captcha').innerHTML = captchaURL;