User blog comment:Brandon Rhea/Wiki Modernization Stage Two: Page and Article Headers/@comment-5091911-20170419085914/@comment-20644-20170508032738

Sorry I missed this comment earlier.

You will be able to change the header image (as well as the header colors) in the Theme Designer. That update to the Theme Designer will be released at the same time that the header redesign is released.

The fading happens automatically as well, so you only need to upload the image and then the system takes care of the rest.