Masalah Upload Thumbnail

Saya sedang membuat file upload gambar menggunakan thumbnail hanya saja saat dijalankan entah kenapa file thumbnail nya nge blank HITAM, sedangkan gambar asli normal.
 
Source:
		$photo		= $_FILES['filename']['tmp_name'];
			$photo_name	= $_FILES['filename']['name'];
			$photo_fix_name = $photo_name;
			$direktori	= "img/$photo_name";
 
 
			// if the image filename is not null
			if ($photo != ''){
 
 
			$file = move_uploaded_file("$photo", $direktori);
			$gbr_asli = imagecreatefromjpeg($file);
			$lebar = imagesx($gbr_asli);
			$tinggi = imagesy($gbr_asli);
 
			$tum_lebar = 100;
			$tum_tinggi = 100;
 
			$gbr_thumb = imagecreatetruecolor($tum_lebar, $tum_tinggi);
			imagecopyresampled($gbr_thumb, $gbr_asli, 0, 0, 0, 0, $tum_lebar, $tum_tinggi, $lebar, $tinggi);
 
			imagejpeg($gbr_thumb, "./img/thumb/". $photo_name);
 
			imagedestroy($gbr_asli);
			imagedestroy($gbr_thumb);
 
 
			mysql_query("UPDATE `pengajar` SET `noinduk`='$noinduk',`namadosen`='$nama',`alamat`='$alamat',`tanggal_lahir`='$tgllhr',`email`='$email',`username`='$username',`photo`='$photo_fix_name',`kelamin`='$kelamin' WHERE pengajar_id='$pgjrid'
			");
 
 
Kira2 dimana letak masalahnya ya?