All the pictures (banner photos on the home page, cover photos for events, chapters etc.) are designed to be responsive. i.e. they automatically resize themselves to fit the screen you’re viewing it on. Images no longer get chopped off just cause you’re looking at it on mobile.
That being said, the pictures will look better if you stick to the following guidelines regarding their sizes
Banner images on the home page: 3:1 aspect ratio (and at least 1350 px*450 px)
For chapters, events, stories: 4:1 aspect ratio (and at least 1200px * 300px)
Pro Tip: To make your pictures look even better, try to use pictures where the subject (highlight/main focus) is towards the center of the image, both horizontally and vertically. This way, it will look best as a cover image.