With every new version of Poser, there is a different intro and credit screen. I don't know about you, but no matter how novel it may appear at first, I don't really want to see it a few thousand times.

If you feel adventurous, locate

Runtime/ui/30000_credits_splash.psd

and save a copy somewhere, or in the same folder under a different name.

The intro splash screen is layer 30510 and you can paint whatever with a 2D program that supports that format.
The "about" screen is layer 30511 but I only occasionally have to look at it, so it remains the same.