神奇的ob_clean()

<meta http-equiv="Content-Type" content="text/html;charset=gbk" />
<form action="" method="post" enctype="multipart/form-data">
<input name="img" type="file" />
<input type="submit" name="sub" value="上傳" />
</form>

<?php
if($_POST[sub]){
$uploaddir = "G:/php upload files/";
if(!file_exists($uploaddir)){
mkdir($uploaddir);
}
$uploadfile = $uploaddir.basename($_FILES['img']['name']);
if(move_uploaded_file($_FILES['img']['tmp_name'],$uploadfile)){
echo "上傳成功！";
}


$img = GetImageSize($uploadfile);

switch($img[2])
{
case 1:
$im = @ImageCreateFromGIF($uploadfile);
break;
case 2:
$im = @ImageCreateFromJPEG($uploadfile);
break;
case 3:
$im = @ImageCreateFromPNG($uploadfile);
break;
}


$logo = 'G:\php upload files\Chrysanthemum.jpg';
$ing = GetImageSize($logo);

switch($ing[2])
{
case 1:
$in = @ImageCreateFromGIF($logo);
break;
case 2:
$in = @ImageCreateFromJPEG($logo);
break;
case 3:
$in = @ImageCreateFromPNG($logo);
break;
}


imagecopy($im,$in,400,30,10,0,'100','111');

$te = imagecolorallocate($im,255,0,0);

imagettftext($im,30,9,0,0,$te,'STXINGKA.TTF',"跨越新生！！！！！！！！！！！");


$new = ImageCreateTrueColor(500,500);
ImageCopyResized($new,$im,0,0,0,0,30,500,$img[0],$img[1]);

ob_clean();//爲什麼去掉這句就無法正常輸出，而是輸出圖像"http....."因其本身有錯無法顯示？？？
header("Content-type:image/jpeg");
imagejpeg($im);

}
?>