When uploading a background image to your Outro at the end of the video, the suggested photo ratios would be:

Wide ratio 1920x1080px (or 1280x720)
​Square 1080x1080 (or 720x720)
​Vertical  720x1280 pixels
Please note, the Outro background can not be resized.

When uploading images as a slide, it would be the same dimension, but you can always zoom in/out right to focus on a specific area of the photo.