if your using the a5 templates, you can alter the splash screen in the main script file (yourgame.wdl), just by changing the bmp you load. so, where you see a line like this:
bmap splashmap = <logodark.pcx>;
or
bmap splashmap = <logolight.pcx>;

just change to the image you want (i believe that if you have pro edition, it's ok to do this):
bmap splashmap = <your_image.bmp>;

in the a6 templates i'm not sure how it works...