Whether a private tree is indexed or not has no bearing on whether media in that tree should be viewable in thumbnail in search results. From the info you clipped, name, birth year and birthplace is the only "limited" info that should be displayed. The fact that private media turns up in a search is not the issue (that has always been the case). The issue is that a thumbnail of the image is being displayed - when ancestry clearly states that media in private trees is not veiwable to anyone without access to the private tree.