Hello Danny, For image center alignment add the following codes in rtMedia custom CSS box.
body .rtmedia-container .rtmedia-list .rtmedia-list-item, body .rtmedia-activity-container .rtmedia-list .rtmedia-list-item, body #buddypress div.rtmedia-activity-container .rtmedia-list .rtmedia-list-item { width: 100%; }
body #buddypress div.rtmedia-activity-container ul.rtmedia-list li .rtmedia-item-thumbnail { margin: 0 auto; }
About other 2 question, We haven't added this functionality yet, But if you want to add it, you need to add some jQuery codes.