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. Just drag the uploaded image to your canvas.
You can then use the Arrange function in the tool bar at the top: set your uploaded picture as the most bottom layer by clicking "send to back."
Note that there are 4 layer buttons in the Arrange tool above your canvas on the top tool bar. Select the picture which you want as a background image, and click on the second - “send to back” - Arrange button and it will be layered at the bottom, acting as a background on the block.
(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 upper menu). Another helpful 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.