Editing backgrounds

Evening, 

I have a strange question, which hopefully I will explain correctly. To most slides (those that include a click box) 2 possible slides.

The main slide has a background that you can select "Find Background in the Library", which you can then click "Edit" in the Library pane.

There seems to be another background associated with a slide that gets triggered when the timeline hits the click box.

To show this the main image is below and through the timeline this is what is shown, and when I got to edit the background, this is what I see.

When the timeline hits the click box the highlighted image below appears and the change of colour. I cannot find a way to edit this image (if it is an image).

Is it possible to edit this image, and if so how? 

Apologise for any wrong terminology.

Thanks

Phil.