Showing thumbnail only on the "last" gallery

Imagine you have gallerie 1(root), gallery 2 inside gallery 1, and gallerie 3 inside gallerie 2. Gallerie 3 as an image inside of it (the other 2 are empty, they don´t have any images). When we look at gallerie 1 we will see a thumbnail of that image (the one on gallery3). I need UrgentlY! to see this thumbnail only when looking at gallerie 3. How can I change the code in order to avoid galleries of a lower level to go get a thumbnail from higher levels?

thank you.