While I can understand the claims that he has a "generic image", that question is pretty easy for a djoko fan such as Nina and I.
Young, rebel and and entertainer.
See, this is the problem. In the eyes of the casual tennis fan, everything about his image contradicts that.
Young? Nope. Here's a photo of Nadal and a photo of Djokovic, both from last year's Australian Open:
Show a random person with no knowledge of either player and ask them who's younger, and I guarantee you that every single one will say Nadal - even though he's a year older. The hair, the clothes - it's all completely unfashionable. Seriously, add a cap with a neck flap to that picture and you've got Ivan Lendl circa 1989.
Rebel? What about him is rebellious? He's never pushed the boundaries of fashion like Agassi. He doesn't have the reputation of being a brat like McEnroe. He's never got up the nose of the establishment at Wimbledon. His interviews are polite and politically correct. He even has the sort of hairstyle they give you when you sign up for the army!
Entertainer? How? To the casual tennis fan, his style his very boring. He doesn't have the jaw-droppingly elegant strokes of Federer. He doesn't have a distinctive picture-perfect signature shot like Nadal's lasso forehand. He makes his little jokes, like his impersonations of other players and his skit with Leconte at the AO, but only die-hard tennis fans see or care about this stuff. It's not something you can capture in a photograph or a TV commercial.
I know the question's not for me, but here's my two cents:
Djokovic's image: the ironman of tennis, a tough competitor, warrior, gladiator who will leave everything he has while on battle, and yet he can be funny, warm, playful and charismatic off court.
The bottom line is that none of this stuff is conveyed by the way he looks. Nadal LOOKS like a tough, primal competitor. Federer LOOKS like a suave, sophisticated charismatic guy.
Djokovic might be tougher and more charismatic than both put together but he sure as shit doesn't look like it. In fact he mostly just looks like a regular guy who happens to be really, really good at tennis.
This is the problem. For 99% of consumers, personality doesn't equal image. Djokovic's image doesn't make people want to aspire to be anything, and as such he is a pretty poor commercial option (relatively speaking).