How to set a target as image in given code.

 public void addListenerForImage(final Image roomImage)
     {
         final DragAndDrop dragAndDrop = new DragAndDrop();
         dragAndDrop.addSource(new DragAndDrop.Source(roomImage) {
             public DragAndDrop.Payload dragStart (InputEvent event, float x, float y, int pointer) {
                 DragAndDrop.Payload payload = new DragAndDrop.Payload();
                 payload.setDragActor(roomImage);
                 dragAndDrop.setDragActorPosition(-x, -y + roomImage.getHeight());
                 return payload;
             }
             public void dragStop (InputEvent event, float x, float y, int pointer,Target target) {
                 roomImage.setBounds(50, 125, roomImage.getWidth(), roomImage.getHeight());
                 if(target != null) {
                     roomImage.setPosition(target.getActor().getX(), target.getActor().getY());
                 }
                 System.out.println(target);
                 stage.addActor(roomImage);
             }
         });

My problem is i can drag the images and i am not able to set target as image; and target shows as null;One more if a invisible some of the images in group how can i test that it is overlapped or not;Please give some links and suggestion