<?PHP    
$img = file_get_contents('http://www.site.com/image.jpg');

$im = imagecreatefromstring($img);

$width = imagesx($im);

$height = imagesy($im);

$newwidth = '120';

$newheight = '120';

$thumb = imagecreatetruecolor($newwidth,$newheight);

imagecopyresized($thumb,$im,$newwidth,$newheight,$width,$height);

imagejpeg($thumb,'/images/myChosenName.jpg'); //save image as jpg

imagedestroy($thumb); 

imagedestroy($im);
?>

关于PHP图像功能的更多信息：http://www.php.net/manual/en/ref.image.php