When creating a new post, you might want to think about the size of the image uploaded to that post, and how it will appear in the Content Library. In order to ensure that your image will not be cut off on desktop, you can choose an image that has a 2:1 ratio, where the width is twice the size of the height.
If you are looking to optimize your image for the mobile app, the image can have any ratio where the width is equal to or larger than the height. Anything ranging from a perfect square to any type of landscape image will fully appear in the mobile app. Portrait style images will get cut off on both the mobile app and on desktop.