You need the Campaign of Carnage DLC for this.

Whilst in any of the new locations introduced in this DLC, there is a small chance that any enemy with the words 'Bad-ass' before its name will drop a framed pictures of Moxxi, upon dying.

The picture has a purple glow when it has dropped (Looks like PURPLE rare loot), and is a rather large, rectangular, somewhat unmissable object, amongst all the weapons and cash dropped from dead enemies, so you won't miss them when they drop.

Fastest way to farm these is at 'Pyro Pete's Bar', which can be accessed by travelling through 'The Beatdown' area. A lot of 'Bad-Ass' enemies spawn here if you activate the mission, 'Bar Room Blitz' within this area and you can activate the mission as many times as you want. Just start the mission, kill everyone inside the bar, including the large amount of 'Bad-Asses' that spawn here, hoping that 1 or more will drop a picture, if so pick it/them up, then save and quit. Then hit continue when you get to the main menu, and you will already be inside Pyro Pete's Bar when you spawn. You can then activate the mission again and continue the cycle until you have found 10 pictures to unlock the achievement.

*Note; This achievement is reportedly glitchy and may not unlock for some people after having found 10 pictures. Do not worry though, as it will unlock, you may just need to find more pictures. It may take somewhere in between 10 and 20 pictures to unlock the achievement, but it will unlock. Some report that there are 10 different pictures that must be collected. This is currently unconfirmed.