Step 1: Within the editor, go to the desired page where the background needs to be changed. Then click on the "style" pencil icon to bring up customization options.
Step 2: In the "Background" panel, here is where you can either select/change an image to be placed as a background.
Step 3: Click on the "Select" plus icon.
a) Here is where you can choose an image from your library, or...
b) Upload your own image, or...
c) Search for an existing image from the web.
Step 4: After selecting an image, click "Confirm" to apply.
Step 5: Voila! You'll now see that your background has been set.