It all started on MTV's "Pimp My Ride" show.
There, this guy nicknamed Xzibit shows "pimped" dudes/gals their new cars saying "Yo dawg, we heard you like...", following of something dudes/gals like to do. i.e.: "Yo dawg, we heard you like cars, so we put a car in your car so you cand rive while you drive".
It doesn't make much sense, until we extrapolate it to the internet world. Now a gazillion pics have emerged, from image board users such as 4chan, with very very creative twists.
They're usually 1 framed pics, with a montage of Xzibit over another pic, but there are variations, some of them have 3-4, or even more frames.
Multiple frame pictures sometimes depict Xzibit in different moods, in kinda reaction to other pics he's mixed with. I find them extremely hilarious.
I don't know dude, it just is.