captcha อัพขึ้น host ไม่แสดงภาพ มีวิธีแก้ไขอย่างไรครับ

<?php 
session_start(); 

function _generateRandom($length=6) 
{ 
    $_rand_src = array( array(48,57), array(97,122), array(65,90)); 
    srand ((double) microtime() * 1000000); 
    $random_string = ""; 
    for($i=0;$i<$length;$i++){ 
        $i1=rand(0,sizeof($_rand_src)-1); 
        $random_string .= chr(rand($_rand_src[$i1][0],$_rand_src[$i1][1])); 
    } 
    return $random_string; 
} 

$im = @imagecreatefromjpeg("image/captcha.jpg");  

$rand = _generateRandom(3); 
ImageString($im, 5, 2, 2, $rand[0]." ".$rand[1]." ".$rand[2]." ", ImageColorAllocate ($im, 0, 0, 0)); 

$rand = _generateRandom(3); 
$_SESSION['security_code'] = $rand; 
ImageString($im, 5, 2, 2, " ".$rand[0]." ".$rand[1]." ".$rand[2], ImageColorAllocate ($im, 255, 0, 0)); 

Header ('Content-type: image/jpeg'); 
imagejpeg($im); 
ImageDestroy($im); 
?>