There are a total of 8 Posters in The Walking Dead: Survival Instinct. Finding all of them will earn you the "The Missing 8" trophy / achievement. Your progress will be saved everytime you find a new collectible. There is no need to look for it again if you die.

WARNING: You cannot get all collectibles in one playthrough! You have to play through the story at least two times. At some points you can travel in two directions with your car. If you choose one location you won't be able to visit the other unless you start a new game and replay the areas. All statistics (collectibles, kills etc.) will stay if you start a new game in the main menu.