You can try to query the image directly from the database:
global $wpdb;
$query = "SELECT ID FROM {$wpdb->prefix}posts WHERE post_name LIKE '{$img_name}' AND post_type LIKE 'attachment'";
$thumb_id = $wpdb->get_var($query);
if( !is_null($thumb_id) ) {
$attachment = wp_get_attachment_image_src($thumb_id, 'full');
$img_url = $attachment[0];
}
The question was posted more than 1 month ago, so I don’t think it will be useful for author. But I hope it will be helpful for someone else.