you could just remove the if statement altogether, you know it must be greater than or equal to 100 after the first for loop.

preferably i'd use a while loop
Code:
while(splash.alpha < 100){
  splash.alpha += 5*time_step; //change this to change the speed
  wait(5); //change this to 1 otherwise you'll get uneven fading.
}

wait(-1); //keep onscreen for 1 second
while(splash.alpha > 0){
  splash.alpha -= 5*time_step; //as above
  wait(5); //as above
}
though the result would be the same