>why would a quote need an image at all?

Need is a bit of a loaded word. But I think it's obvious how a picture (which is proverbially worth a thousand words) could reinforce the message of a quote.

>And if so, why not use an image of the person who wrote the quote?

Because what's to say that's the best image to reinforce the message of the quote? It's fair to say using a portrait of someone else is a bit odd. But considering it's a fictional character who's story almost perfectly reflects the message of the quote it makes a lot of sense.

The character of John Wick walked completely away from a life of violence, until violence randomly found him again. Then he fought back with everything he had. The arc of his story is an excellent match for the quote.

>How would you feel if someone put a picture of you behind this quote?

John Wick is a fictional character. It really doesn't matter how he feels, because he doesn't exist. And it doesn't matter how Keanu feels, because we're supposed to understand that this is John Wick as portrayed by Keanu, not Keanu himself.