Yup just create your image and save it as gimp-splash.png in this directory
C:\Program Files\Gimp-2.7.3\share\gimp\2.0\images
make sure you take the other one out first in case you want it later.
If you want to keep it with all installs of 2.7 -
Install the gimp-splash.png in your YOURUSERNAME/gimp-2.7/splashes folder create one if needed.
Remove the one as said before.
When Gimp-2.7 loads it will load the image in your splashes directory.
You can add as many as you like there and Gimp will open them at different times.