Enter (U,U,D,D,L,R,L,R,B,A) at the title screen. You will hear and see the turtles yell cowabunga to confirm it has worked.
From the "Options" menu select "Input Password" and enter the following.
||Crystals become Easter Eggs
||Crystals become Jack-O-Lanterns
||Crystals become Santa
||Life Icons become Pizza
||How to Unlock
|ALternate ending Picture
||Collect all gems in Story Mode
||Complete mission mode with Donetello
||Complete mission mode with Leonardo
||Complete mission mode with Michaelangelo
||Complete mission mode with Rapheal