To add a background from the Content Library to a slide, do the following:
First, click the Backgrounds button on the toolbar.
Next, enter search terms at the top, or narrow down your criteria on the left.
Then, choose a background from the list.
Finally, click the Insert button. The background you have chosen will be inserted onto the slide. You can change the size of the image, place, rotate, flip or crop it the way you like.