Cheat Codes:
At the main menu highlight "Versus" and then, in this order, press and hold L, B, R, and then, in this order, release B,R,L. If done correctly, a cheat menu will appear. Enter the following codes to unlock these cheats. If you entered the correct code, Godzilla will roar. 

Effect: Code 

12 Contnues in Adventrue Mode: 548319
Add A.I. Player to Melee: 154974 
All Cities: 107504 
All Cities and All Monsters: 863768 
All Gallery Items: 962129
All Monsters: 209697 
All Monsters Are Invisiable: 600225 
All Players Are Small: 154974 
All Players Deal Quadruple Damage: 817683 
All Players Take No Damage: 569248 
Black and White Mode: 860475 
Boxing Ring: 440499 
Constant Energy for Player One: 778393 
Constant Energy for Player Two: 881557 
Constant Energy for Player Three: 597558 
Constant Energy for Player Four: 218967 
Destoroyah: 352117 
Disable HUD: 880460 
Display Game Version Number at Option Menu: 097401 

Unlock Hints: 

Get King Ghidorah: Beat Adventure mode with Megalon.
Get Gigan: Beat Adventure mode with Angilas.
Get Destroyah: Beat Adventure mode with Godzilla (Millennium).
Get Rodan: Beat Adventure mode with Gigan.
Get Mechagodzilla: Beat Adventure mode with Rodan.
Get Mecha-King Ghidorah: Beat Adventure mode with King Ghidorah.
Get Orga: Beat adventure mode using all characters then beat the adventure mode one more time using Godzilla 2000. 

Unlock Pictures In Picture Gallery:
To unlock pictures in the Gallery, you must play through Adventure Mode on the Medium or Hard difficulty setting. Some buildings that you destroy will reveal Silver Atari symbols. Pick these up to unlock a picture.