With the clone tool active, change the opacity in the top menu bar.
I'm not sure I understand exactly what you're asking about opacity and ghost images. However, if you are clone stamping, it will never go behind other pixels (images) on the layer to which it's applied. The clone stamp goes over what's already there.
That said, if the opacity of your tool is set low enough, it may appear to go behind, even though it's really on top because the transparent marks only show up clearly on the background. For instance, on the example you provide, it could seem that the cloning of Minnie Mouse was going behind simply because it's so transparent that it's barely visible on the solid Minnie whereas it shows up clearly on the white surface surrounding her.
Let us know if that answers your question or if you need further assistance.