I have no way of knowing if it makes a difference in the numbers, really.
All I know is that I do like to change my profile picture frequently --- mostly because I just get tired of looking at it after a while.
Also, I've sometimes wondered why other Hubbers don't change THEIR profile pictures, because I get sick of looking at the same old thing every time I read one of their Hubs.
Of course, it's not really a deal-breaker. If I like their Hubs, I wouldn't change my mind or anything based on the profile image.
I think JM's answer is also good.
So, I think the bottom line here is: it's up to you. Just try out different graphics and things, and see how you and others feel about it.
I do look for some nice "eye candy" on a profile, and it's nice when a Hubber provides that, I think. It reminds me of the gracious folks who spend so much time and money decorating their homes for the holidays, so that others can enjoy the view as they drive or walk by. It's kind of a little gift to the universe.
I've also been driven away immediately by an ugly, nasty profile graphic or picture. I don't mean anyone's face is ugly --- just something offensive, nasty, boorish or obnoxious.