For information about adding a slide and changing its aspect ratio, see this article: Adding and Modifying Slides
To set a slide background, you must first upload your desired image to the Alleo board. You can do this through many methods including the following:
Drag a file straight from your computer's file explorer into the Alleo board space.
Click the plus sign in the Files panel and choose a file from your computer.
Copy a file from the web or your computer and paste into your Alleo board.
Search for an image using the Board Search function on the left of your header bar. Drag in an image from your search.
Click on the header of your desired slide and click on the three-dot menu to access the Object Settings.
Go to the Theme tab and click on the image icon to select an image
Select your desired image by clicking "Select" underneath the thumbnail of your image.
Click "Save".
