By visualizing size needs of all these sections, the most appropriate size for Facebook Event Photo is determined as 1920×1080 pixels with 16:9 ratio. If you have images wider than this, they will be cropped from sides and the taller images will be automatically cropped from top and bottom; so it is good to choose the right size for uploads.

https://photo.wondershare.com/photo-editing-tips/facebook-event-photo-size.html