To change the background photo, open up the main menu by clicking the three verticle dots in the top right of the page. Then select "settings" from the dropdown. On the left side of the settings screen, select "background" and then choose the photo you'd like.
If you'd like to upload your own photo, select "custom photo" and enter the url of the image you'd like as your background. (Tip, you can upload and store photos on sites like imgur for free!)