No format-page option
Click on button “Master Slide” in the properties deck in the sidebar or switch to Slide Master in the menu View.
You can set the background itself to a bitmap filling; that is in item Slide Properties > Background in menu Slide or in context menu. Or you add an image as background object to the slide; that is menu Insert > Image.
In addition, there exists an item “Set Background Image” in menu Slide or context menu and a button “Insert Image” in the sidebar. That works too, but it stretches the image automatically to fill the slide and you cannot access other option like Tiled or enlarging with keeping ratio.
When done, close the Master View or switch back to Normal View.
Master Slides are recommended for backgrounds. But you are able to set the background of a single slide in similar way.