To add your own JPEG or PNG image as a background, you first need to upload the image to your account. You'll use the Uploads tool in the left side panel to do this. Then drag the uploaded image to your canvas.
You can then use the Arrange function to layer your image to be the background (found in the tool bar at the top): set your uploaded picture as the most bottom layer by clicking "send to back."
Optional: If you want the image to stay in one place while you edit other graphic elements, you can use the lock tool (lock in tool bar at the top).
Pro Tip: Your cursor will turn into a two-way arrow when you mouse over the image handlers. Hold on to the SHIFT key on the keyboard as you click and drag. This well preserve the ratio of your image as you move the corners of the image to enlarge or shrink it.