I wanted to create this solution to cover a few misconceptions regarding this achievement
The pictures of Moxxi can be dropped by enemies throughout the entire DLC, however it is unlikely by the time you have completed the main storyline, you will have found 10.
The best location to farm the pictures is at Pyro Pete's Bar. You just need to walk into the bar and attack an enemy in order for a brawl to break out. The Badass Brawlers are most likely to drop the pictures of Moxxi. I found that more Badass Brawlers spawned whilst just walking into the bar, having not started the Tier 3 Battle: Bar Room Blitz Mission, then when starting it. Once the enemies in the bar stop spawning, simply walk back to The Beatdown entrance and then back in to reset them all.
The pictures will continue to drop once the main storyline has been completed. This achievement is all about RNG. A few people will get lucky and find the pictures quickly, for the majority it will take a few hours of grinding.