How to upload custom header to SharePoint online Site?
Here are the steps to add custom header for SharePoint Online Site
Choose the "no image" theme.
Choose the 3rd style from the bottom from Styles dropdown list
Remove any header text and click on Logo tab. Now upload your new header image. Make sure the image width is 978 (Default size)
When you upload the image, make sure you uncheck optimize my image. If optimize image selected, you image will be uploaded as small image
Once your image is uploaded select your new image as header
Your new header should look like this in SharePoint Online site. This is how you upload custom header to SharePoint online Site