The tooltip description of the Repository Branding Image URL isn't exactly clear:
A URL or other file location identifier referencing a file that contains a branding device to be used in the repository's online finding aids. A typical branding device is a university seal or logo.
Maybe something more like this:
A remote URL or path to a local file that contains a branding image (like a logo or seal) to be used in the repository's online finding aids. This image appears on the public side PDFs and on the repositories page.