Background controls are found in the upper settings menu of the editor. This will open the background editing panel with these options: Color Set for the slide, Background Style and Background image.

Changing Color Set will apply a different color variation to the slide background and elements, alternating the color combinations available in the palette. 

There are different Background Styles you can apply, with gradient and shadowing effects. 

To add a Background image just hit the "Select an Image" button. You can browse images by just typing in keywords or upload your images.  

GIF animations and Icons can be used as backgrounds too, you can select this by changing the input on the left side of the browse window. 

Background images can be dimmed and depending on its dimensions, you can choose to fill the slide with it or crop it to keep its original aspect.