FileRun provides a control panel option for setting the URL of an image file to be used for the login page background.
That URL can actually point to a script which redirects to various or random images.
Here's a great website with free beautiful photos which can be used https://source.unsplash.com
For example, use this URL to get random backgrounds from the above website:
The following image is loaded like that. Just reload this page to see another image: