Successfully unlock SuperStiff mode in each level to unlock the corresponding bonus:

Behind the Scenes pictures: Level 1
Nikki: Level 2
Hey, Ladies! video: Level 4
The Girls of Summer pictures: Level 5
Monique: Level 6
Mind Fuck video: Level 7
Eye Candy pictures: Level 9
Simone: Level 10
Back Stage video: Level 11
Got Titty pictures: Level 13
Mandy & Mandi: Level 14
Meet Matt video: Level 15
The Fruits of Our Labor pictures: Level 17
The Babes video: Level 18
Night At Padre video: Level 19
Game Over! video: Level 20
Boobs, Babes and Boobs pictures: Level 20