Go to your Runtime and in the folder "ui" you'll find this file: 30000_credits_splash.psd. First make a back-up copy in case something goes wrong. Then, open it in any 2D editing program that supports .psds and layers and look for the layer with the number 30510. Make an image of the same size and replace that layer with your image. Make sure you name it 30510 as well. Save and open Poser!