Restrict download by role

function king_add_download($text){

    global $post;
    $user = wp_get_current_user();
    $thumb_URL = get_the_post_thumbnail_url( get_the_ID(), 'full' );
    $download_class = "_YOUR_CLASS_HERE";

    if(in_array('administrator', $user->roles)){

        if(($post->post_type == 'post')){

            $last_tag = strrpos( $text, "</" );
            $text = substr_replace( $text, '<div="'.$download_class.'"><a href="'.$thumb_URL.'">Download this Image</a></div>', $last_tag, 0 );
        }

    }
    else{

        $last_tag = strrpos( $text, "</" );
        $text = substr_replace( $text, '<div="'.$download_class.'"><a href="#">Download this Image</a></div>', $last_tag, 0 );
    }

    return $text;
}

add_filter('the_content','king_add_download', 1);