Actually, it's "I am your father," because using the contraction for "I am" simply doesn't have the same impact. If Darth Vader ever said "I'm your father" to Luke in The Empire Strikes Back, I suspect the moment might not have been as profound. When James Earl Jones speaks and draws out those words it's potent. I imagine a good deal of thought went into that singular moment. Actor David Prowse had no idea what the dialogue would be for that definitive moment until much later.
Still, that image is pretty clever.