Enter the codes at the password screen, and you'll get a message confirming they work.

Password	Effect
Frogger, Frogger, Frogger, Frogger	Hidden Image 1
Finnius, Finnius, Finnius, Finnius	Hidden Image 2
Berry, Berry, Berry, Berry	Hidden Image 3
Dr Wani, Dr Wani, Dr Wani, Dr Wani	Hidden Image 4