thanks for posting your idea to the Forum!
At the moment the paper is always the background layer. It is not possible to place an imported image behind the Canvas paper.
It is only possible to import the paper to a separate layer, hide the actual Canvas paper in the Layers panel, then place the layer with the imported image below the paper on a separate layer.
May I ask what you would like to achieve by placing the image behind the paper? It'll help us understand your idea better. Any examples are welcome, too :)