I've managed to add an image as a site icon for a sub-site, although the scaling algorithm didn't behave as expected and only the middle of a wide logo appears. That in turn seems to have also set all the config screens to black on black. The pages users can navigate to appear fine with the site logo partially visible. O365 SP will now not let me save a different icon that I scaled manually, and I can't seem to remove it or set it back to the default. It's not an option under look & feel, change the look or site information screens (see screenshots).