You need to download
http://www.ppcgeeks.com/files/apachesplash_154_177.zip
It will let you do what you are looking for. I have never tried with a custom picture but I think that so long as the dimentions are correct, and the extension is correct, it should be point and click.
I know it was super easy to go back to the Sprint logo after the screen changed to "Telecom" when I went to the custom 3.5 rom.