When you drop an event or page, you can add header images. The header image will be cropped to a size of 1112x279px, which is approximately a 4:1 ratio. The same image will be displayed in the feed when you publish your page. Here the image will be cropped to 730x474px (3:2).

The graphic below shows the area which will be visible in the page header as well as in the feed. The photo has a ratio of 4:3. The purple frame shows the crop for the feed. The flamingo colored frame shows the cutout for the header.