Alright, so you found an easter egg of some sort....
You know, there have been other games (MANY OTHERS) that contain easter eggs. Hell, even your OS has easter eggs on it.
I know in other games sometimes there are pictures of characters, faces of creators, interns faces being used on dead players, swear words, random scenes from pornography (both anime and real life), and sometimes characters from other games from the same developers, all being displayed somewhere within the game.
Easter eggs are common everywhere, even as little as being a character or picture of something that can only be seen under a high magification microscope, like on the circuit boards in your PC. (Go ahead and just RIP them out to take a quick look!!)
Um, there really aren't any reasons to place easter eggs in programs, except that an employee can do it. I know most software development companies do not approve of this practice, but what the fuck?, people get bored at their jobs. SO why not have some fun...
Figure there's one person at every job, why not post their pic on some zombie layin' in a gutter with most of it's body eaten away??