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<?php
   header("Content-type: image/png");
   list($width,$heigth)=getimagesize("image.jpg");

   $image=imagecreatefromjpeg("image.jpg");
   $tn=imagecreatetruecolor(50,40);

   imagecopyresampled($tn,$image,0,0,0,0,50,40,$width,$heigth);
   imagepng($tn,"test.png");  //speichert den inhalt von $tn (thumbnail) unter test.png

   imagepng($tn);            //zeichnet das thumbnail im browser
   imagedestroy($image);
   imagedestroy($tn);
?>