This is my method for getting Gold on the PP photo Challenge: "Shutter Bug" (single player) and "Picture Perfect" (co-op). Before heading to Uranus Zone to do the challenge collect all three photo PP magazines. These add to your PP% for every picture you take.
1) Erotica mag, (as mentioned by count023) Slot Ranch Casino in Mens Bathroom. adds 100% to erotic photos.
2) Photography 2 mag, located in Palisades mall on second floor in Robsaka Mobile store. adds 10% to all photos.
3) Photography 1 mag, located in Uranus Zone in First Aid room by farthest right maintenance room. adds 10% to all photos.
Note: All three magazines should be held by one player. Might also be able to do this challenge with only Erotica mag.
Other useful items for this challenge: Quick Step, Repulse, and/or Untouchable juice, and the Ninja outfit. These will help in avoiding zombies which can knocking you out of camera mode, however they are not necessary.
Before starting the challenge locate the Souvenirs booth next to the Molemen from Uranus mini game. This is found straight across the walkway from the bar Jump Space 7. Once inside the Souvenir booth grab and place many (roughly 30) Alien Probes on the floor. This spot will spawn these infinitely.
After the floor is covered, start the challenge run back to this booth and stand on the counter to photograph the floor.
This method literally only required one photo from my co-op partner and one from myself having all three books in my inventory. Gold medal in one picture on single player too. My highest score using this was 57366 PP in single player.