The wanted posters are spread in some of the campaign levels. Mostly inside buildings.
Below find a video with the location of all posters. This video was not created by me.
If you collected a poster you can not just quit out. If you do, progress is probably not saved.
So either finish the mission your are playing or fail on purpose for the game to save.
Every poster unlocks a new challenge in the respective tab. There are 10 challanges in total.
Completing 50% and 100% of them unlocks those two trophies.