Go to Side Panel -> Colors -> Background to upload a background image. The canvas size will be automatically adjusted to the size of the image.
The background image cannot be moved or recolored. It is perfect if you want to add a world map, city map, or family tree as the background.
If you want to have an image in the background that you can move, it is better to add it through Side Panel -> Tools -> Uploads. You can lock this image via Side Panel -> Layers.