Cita:
Iniciado por bet7o 
si le mandas las cabeceras?
No entiendo a que te refieres.. mira, este es e mi codigo :
Código PHP:
<?
require_once("mf.php");
fnSessionStart();
$RandomStr = md5(microtime());// md5 to generate the random string
$ResultStr = strtoupper(substr($RandomStr,0,5));//trim 5 digit
$NewImage =imagecreatefromjpeg("base.jpg");//image create by existing image and as back ground
$LineColor = imagecolorallocate($NewImage,233,239,239);//line color
$TextColor = imagecolorallocate($NewImage, 163, 148, 148);//text color-white
imageline($NewImage,0,0,40,40,$LineColor);//create line 1 on image
imageline($NewImage,40,0,80,40,$LineColor);//create line 1 on image
imageline($NewImage,80,0,120,40,$LineColor);//create line 2 on image
imageline($NewImage,120,0,160,40,$LineColor);//create line 2 on image
imageline($NewImage,160,0,200,40,$LineColor);//create line 1 on image
$font = 'stampa.ttf';
imagettftext($NewImage, 16, 0, 63, 30, $TextColor, $font, $ResultStr);
$_SESSION['key'] = $ResultStr;// carry the data through session
header("Content-type: image/jpeg");// out out the image
imagejpeg($NewImage);//Output image to browser
?>