Loosely defined, “In your base, killing your d00ds” is a catchphrase that basically means “right now, I’m pwning you, and you don’t even know it."
The phrase originally came about when a ‘N00b’ player, playing multiplayer Starcraft online typed in a message asking his opponent “dude, where are you?” At the time, the more experienced player had managed to rush into the N00b’s base and had started destroying everything in sight. His response is now famous: “I’m in you’re your base, killing your d00dz.”
Screenshots of this famous exchange were posted on imageboards, and the meme spread from there.
Initially, people took the phrase and placed it on a number of different pictures. But as the meme grew, the phrase took on a life of it’s own.
The formula: take a picture and add any variation of "Im in ur (noun), (verb-ing) ur (noun)".
During the height of the LOLcat meme, it became very popular to put an “im in ur” phrase together with a cat picture.
Political images were also very popular at one point. The meme is quite old now, but you still do see references to it popping up from time to time.