I can't see it really negatively impacting (aka some adCom saying 'oh geez not another picture, forget about this guy') you, but I agree in terms of you should only include it if it greatly explains/clarifies something that the readers of your essay(who are very smart, capable people) would not be able to normally understand, even if you gave a verbal explanation. Situations like what Gil listed in his post. And it better be something really unique, not just something that many applicants could include.
Interestingly enough, I believe Stern lets you send in some sort of visual presentation (rather open-ended) for one of their 'questions'. That leads me again to think that adComs definitely ARE looking for some sort of creativity, so if you really think said picture is something that will make someone say 'hmmmm, interesting' I'd say go for it.
Not sure if its comparable but I found myself in a similar boat wanting to include a footnote with a hyperlink that explained a couple of the small businesses I have worked with in Peace Corps (that I was 100% sure the adCom's reading my application had not heard of). I ended up asking said school if it was ok to which I received 'if you think it strengthens your application' ) and ended up including it - and got an interview invite. Take that how you will.
As always just my $0.02.