hehe...it`s easy: find 4 files named "splashc1.tif", 2, 3, and 4 and replace them.
(Your install folder + \Terragen-4.ucr\Generic). Be sure to make backup copies!
I simply reworked the copies of the original files in Photoshop and put them in there instead.
Now, everytime I start up TG I get one of the new splashscreens.
cheers, Klaus
