The canvas is the main workspace, the background onto which all other elements will be placed.

An image or paper texture can be  be assigned to the background frame by dropping it onto the grey area around the canvas.
 
If the image is already in a normal frame on the canvas, right-mouse select it and "Set as Canvas Background", or pick it up by the frame number and drop it outside the canvas. The image (or texture) will snap to fill the entire frame.

To move the image to the canvas as a normal frame, right-mouse select an empty space on the background image, and select "Set as Regular Frame".

This section will teach you how to change the size and appearance of your canvas: