Here are the folders and places you go in to, respectively:
There, you will find an image called, "gimp-splash". That is your start-up screen image. To use a custom image of your own, copy the dimensions by clicking on the image's properties. Use those dimensions when your making your custom screen in GIMP. After you're finished with your design, save it as "gimp-splash.png" or over-write the default start-up screen. Then, exit out of GIMP and initiate GIMP again. You will see your custom start-up screen.