Answer
The URL path used in Branding options is most likely pointing somewhere the client browser can't reach without authentication — login-screen images have to be accessible with no login required. Copy the branding images into the Images folder under www-root in the IFS application server's instance directory (<ifs_home>\instance\<instance_name>\www-root\Images); once placed there, they're reachable at <YOUR_IFS_URL>/Images/your_image.jpg without any authentication, which is what the login screen needs. Keep the image files reasonably small/optimized too, since a large image adds noticeably to how long the login page takes to first load (at least before the browser caches it).