With the Slide image & thumbnail option, you are able to set a background image for the current slide, and also able to set another image if you would like a different one for the thumbail navigation. Click on the image to open the Media Library to choose or upload an image. You can also use dynmaic image there with the generator feature. You can learn more about that here: Dynamic slides.
Under the Slide image settings you can set the image size and position on the canvas. Here are them in details:
Auto: Default value. The background-image contains its width and height.
Cover: Scale the background image to be as large as possible so that the background area is completely covered by the background image. Some parts of the background image may not be in view within the background positioning area.
Contain: Scale the image to the largest size such that both its width and its height can fit inside the content area.
Stretch: Stretch the image to fill up the slider. This setting won't keep the image ratio.
Color parameter can add background color for the slide.