Mine is default for 2.7.2, I don't know how to change and probably don't need to
It's really easy, it just takes a little explaining. In windows the splash is stored at C:\Program Files\GIMP-2.0\share\gimp\2.0\images. I recommend copying or renaming your original splash which is named gimp-splash.png. Then make your splash, name it gimp-splash.png and save it in place of the original. then when you restart gimp your new one will be there. There is a way to rotate splashes but i don't know how to do it.
Linux is basically the same process just a different path. i don't have Linux on so I don't off the top of my head know where it's stored but someone will tell you I'm sure. I have a dozen or so that i swiitch now and then. Right now as you can tell from my splash and sig I'm in my gold period.
Linux has the gimp-splash.png file stored here:
but I may make a new one and place it here:
I've noticed that everyone's splash image is a different res. I take it that's ok?