A very cute picture . . . but I must admit I'm confused (I haven't read DH yet.) The doe patronous is . . . Lily's? And it was transferred to him when she died?
D) why'd he have to die *cries*
Okay I'm going to be the nerd here and answer your question Snape's patronous was a doe because he loved Lily. (Her patronous was a doe as well.) Great picture aecr!