PANEL* splash_screen =
{
  bmap = splash_jpg;
  layer = 2;
  flags = SHOW | TRANSLUCENT;
  alpha = 100;
}

void splash_screen_remove()
{
  wait(-3); //wait 3 seconds
  while (pan.alpha > 0)
  {
    splash_screen.alpha -= 10*time_step;
    wait(1);
  }
  ptr_remove(splash_screen);
}

//trigger the function within the main function
function main()
{
  ...
  splash_screen_remove();
}