The header banner is highly responsive, so their are lots of options for setting it up.
This is 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 in the example below. 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 a much wider format, thus both the top and bottom of the image will trimmed. In the most extreme cases of very large and wide desktop monitors, only 360px of the height will be shown, and the full width.
Our recommendation
A 16:9 format photo with a centred motive, no text, a large space between the motive and the top and bottom edges, and high quality. Picking the right image is just as much an art as science - so try out how it looks if you are not sure - you can always change it.