It is best to pre-edit any photos you use prior to uploading them into your WordPress.
Make sure to resize the pixel side sizes to less than 3000 on the long side. Then when saving, save them as JPG and something less than a 1MB file size.
The pixel recommendation will avoid numerous unknown errors when uploading.
The file size of less than 1MB will help you use less server space, where your store your file data [which you pay for]
You can add or change your header image here.
DASHBOARD>APPEARANCE>CUSTOMIZE>HEADER MEDIA
