Is there a way/best practice to changing the Unity 5.0 “Made With Unity” Splash image for the windows/mac player?
Yep. Buy Unity Pro. It gives you the option to change to a custom splash screen. The free version forces you to display the Unity splash screen. See licensing differences here:
You need Unity Pro.
With Unity Pro: Edit → Project Settings → Player → Splash Image → uncheck: “Show Unity Splash Screen”
Then you can create your own splash screen using the UI components.