Since there were some inquiries about this one… This is actually built-in function of Emacs.
First, prepare a graphic file for it. Any size probably will do as long as it’s small enough that fits on the screen. Default graphic is 270×217, so you may want to choose it. Any format that Emacs supports will do. Perhaps XPM will be the best choice.
Now, go into the customize screen using M-x customize
Typing in fancy splash, and it should filter entries to the single one. If it says Default, flip it to File, then specify full path to the graphic you have prepared earlier, and save.
Then it should be displayed when you go in by C-h C-a.