If you have Jamf (with Composer) then it's slightly less of a pain. I found that it only works with local images, so created a package in Composer to drop the college's wallpaper in /Users/Shared/Background. Then just set the owner to root:wheel and make it read-only for everyone so that the image doesn't go anywhere.
We then have a configuration profile in Jamf that forces the background: 'Restrictions > Functionality > Lock Desktop Picture' with the path set to '/Users/Shared/Background/XyzCollegeBackground.jpg'.
Setting the login screen background is a completely different story, and from what I gather, seems to be impossible. Maybe one day our Mac suites will look as nice as the Windows ones... not that it really matters :)