The header banner is highly responsive. So you will not get a short answer to what format to choose here.
How the header banner is shown on various devices.
Mobile phone: 16:9 format. On fundraising pages, the center of the image will be completely covered by the fundraisers profile image, as you can see on the below example. Project pages and landing pages have the whole image shown in 16:9.
Desktop: the same image as on mobile will be shown, but in much wider format, thus both the top and bottom of the image will not be shown. In the most extreme cases of a very large and wide desktop monitor, only 360px of the height will be shown, but full width.
Our recommendation: A 16:9 format photo with a centred motive, no text, large space between the motive and the top and bottom edges, and a high quality. Picking the right and perfect image is just as much art as science - so try out how it looks if you are not sure - you can always change it.