Captcha

Fala galera tou um probleminha, meu codigo de captcha com os seguintes erros.

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at C:\xampp\htdocs\rede-social\captcha\captcha.php:1) in C:\xampp\htdocs\rede-social\captcha\captcha.php on line 2

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\rede-social\captcha\captcha.php:1) in C:\xampp\htdocs\rede-social\captcha\captcha.php on line 4

JFIF>CREATOR: gd-jpeg v1.0 (using IJG JPEG v70), default quality C $.' ",#(7),01444'9=82<.342C 2!!22222222222222222222222222222222222222222222222222F" }!1AQa"q2#BR$3br %&'()456789:CDEFGHIJSTUVWXYZcdefghijstuvwxyz w!1AQaq"2B #3Rbr $4%&'()

<?php
session_start();

header('Content-type: image/jpeg');

$image = imagecreate(200,70);
$fonte = 'Xirod.ttf';
$corLetra = imagecolorallocate($image,255,255,255);
$corLetra = imagecolorallocate($image,255,0,0);

$tFonte = '50';
$qtLetras = 4;

$palavras = substr(str_shuffle("AaBbCcDdEeFfGgHhIiJjLlKkMmNnOoPpQqRrSsTtUuVvWwXxZzYy0123456789"),0,$qtLetras);

$_SESSION['captchaCadastro'] = $palavras;

for($i=1;$i<=$qtLetras;$i++){
	imagettftext($image,$tFonte,rand(-30,25),(($tFonte*$i)/1.5),$tFonte+5,$corLetra,$fonte,substr($palavras,($i-1),1));
}
imagejpeg($image);
imagedestroy($image);

?>