external urls via custom fields

[ShpendBerisha]

Im trying to something like this one:

$thumb = get_post_meta($post->ID, 'thumb_url', true);
$aq_thumb_field = aq_resize( $thumb, 244, 298, true );

when I use a local image in the custom field value its working fine but with external urls it doesnt work.

Im using a video grabber plugin which adds external thumb urls but it doesnt resize the external images.Thankyou.