saves a black image?

[mcshen]

i use this to save image from the web

$filename="blah.jpg";

$url_file = fopen("http://www.site.com/image/01.jpg",'rb');
	$newfile = fopen("$filename",'wb');
	while (!feof($url_file)) {
		$chunk = fread($url_file,1024);
		fwrite($newfile,$chunk);
	}
	fclose($newfile);
	fclose($url_file);

echo "image saved as <i>".$filename."</i><br>";

but, somtimes it works. but, sometimes, it saves a black image. like a 80k jpg file with nothing. just black.

why is that? how can i fix it?