为什么我的file_put_contents调用创建一个零字节图像？

I am trying to create a site that use amazon api to display book information. However, some images I downloaded is showing 0 byte. The code is running on a virtual serever, so curl is enabled.

For example, this image: http://ecx.images-amazon.com/images/I/51d1Zp4f80L.SL75.jpg

My code is this:

try
{
    $ch = curl_init($coverImageUrl);
    $imgName = "images/".$isbn.".png"; //assume png
    $imgFullPath = $_SERVER['DOCUMENT_ROOT']."/".$imgName;
    curl_setopt($ch, CURLOPT_HEADER, 0);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
    $fb = curl_exec($ch);
    if($fb === false) {
        echo 'something wrong';
        }

    curl_close($ch);
    file_put_contents( $imgFullPath, $fb );
    }
    catch (Exception $e) {
    echo 'Caught exception: ',  $e->getMessage(), "
";
    die("problem with image");
}