Level skip:
Pause game play, hold A + B, then press Left(2), Select.

Hidden picture:
Pause game play, hold Start, then press Right, A, Down, A, Right, B, Right, A.

Hidden picture two:
Pause game play, hold Start, then press Select, A, Left, A, Down, Left, A, Select(2).

Hidden picture three:
Pause game play, hold Start, then press A, B, A, Down, Left, A, Down.