The Custom Cover must be an image file (JPG, PNG, etc.) and not a PDF file.
To fill the most space on the page, consider the following.
Space allotted: 740px x 560px (roughly) for US-Letter
Aspect ratio: 1.333:1 or 4:3 (roughly)
If your image file is larger, it will be resized to fit the space and result in more detail and less pixelation.
- Open Presentation PDF Settings
- Click "Show Cover" to reveal the Custom Cover option
- Click "Upload" to pick an image
You will then be prompted to create a Template and enter a name for it.
Then the image uploader will appear, allowing you to select a Custom Cover image.