In order to get this achievement you have to pick up the pictures of Moxxi that enemies can randomly drop in any area of the Campaign of Carnage. Badass enemies are more likely to drop them but I have had regular guys drop them too. The best way to farm this is to do round 3 of Pyro Pete's bar over and over again until you get all 10. You should only farm this once you have done everything else, as doing the main/side missions you will probably run into enemies who will drop them.