Layers superimposition

I am a beginner desperately looking for the way to have a picture appear superimposed over (triggered) layers  in a result slide and have it move across the slide.

Let's try to summarize the picture : I have built a game consisting of a series of quizzes. Each good answer gives access to a specification sheet. At the end of the game, a result slide sums up the score and shows the sheets obtained. These appear via triggered layers (if score>0). I want to superimpose a picture over these layers. It represents a gift box that gives access to all the sheets.

I have tried with the tab order, with the timeline, with the slide layers menu... in vain.

Any solution ? Thanks a lot!