To add a Background to your slide, go to the Design Section.

Click on the Settings button next to your slide, and then on Background.

You'll find here the option to add images, gifs or icons as your background. 

When adding a Background image, all text on top of it will become white.
You can dim your background by clicking on the Settings button and looking for the Dim Background bar. 

If you added an image, you'll find on the bottom of the editor the option to  adjust it to fill the slide or contain it inside the slide.

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