Hi Marcel and welcome to the forum!
It is not very clear either from your picture, nor from your description, what is actually happening in your slide.
When you say: "the photo slides out of the screen", do you mean that the transition that you apply after your slide, affects your backgrounds too?
When are you seeing this happening? when you work on the show, or in the final output (video) ?
I think that you are showing two backgrounds: the first, is your slide background; the second, is your show background.
Can you confirm that you have that colorful gradient both as a slide background and also as a show background?
Because ,during transitions, Producer shows the show background by default, you see that gradient twice.
You can reset the show background to the default black by going to Show > Show Background > Put a checkmark on Type Solid Color, and choose the color black.