Bør billede mindre

Jeg har et script: lavet af detox
//Thumb
function Thumb($sourcefile, $targetfile) {
$max_width = 640;
$jpegqual = 100;
/* Get the dimensions of the source picture */
$picsize=getimagesize("$sourcefile");
$source_x = $picsize[0];
$source_y  = $picsize[1];
if ($source_x > $max_width) {
    $delta = $max_width/$source_x;
}
$dest_x = round($source_x*$delta);
$dest_y = round($source_y*$delta);
$source_id = imageCreateFromJPEG("$sourcefile");
/* Create a new image object (not neccessarily true colour) */

$target_id=imagecreatetruecolor($dest_x, $dest_y);
/* resize the original picture and copy it into the just created image
  object. Because of the lack of space I had to wrap the parameters to
several lines. I recommend putting them in one line in order keep your
  code clean and readable */

$target_pic=imagecopyresampled($target_id,$source_id,
                            0,0,0,0,
                              $dest_x,$dest_y,
                              $source_x,$source_y);
/* Create a jpeg with the quality of "$jpegqual" out of the
  image object "$target_pic".
  This will be saved as $targetfile */

imagejpeg ($target_id,"$targetfile",$jpegqual);
}//END Thumb

Men jeg får fejlen:
Warning: imagejpeg() [function.imagejpeg]: gd-jpeg: JPEG library reports unrecoverable error: in c:\hp\test\test.php on line XX

Når jeg køre scriptet!

Why gør jeg det?