Transitions

I think Shot Cut is Great , I am trying to convince my wife to use it. She wants to use Shot Cut to make a slide show.Her problem is that she wishes to keep her pictures in the same format, showing a background of her choice. This works O.K until she uses a transition . Then she finds the original transparent colour changes to a solid, and the transition uses the whole screen instead of just fading between the pictures . I am sure I was able to do this, but have forgotten.
Please Help