This has been discussed previously and is in the Ideas category: Custom logo on login screen - #22 by ShaneAU. Different users have different ideas about it. Depending on edition, a version of this is already possible. But you really have to think about where you would want this displayed. The Summary doesn’t show when you launch the application, the Businesses page does on the desktop edition. Or the login screen shows on server or cloud editions. And you need access to things just to get started.
Maybe a quick-interval screen saver is what you really need.