Not to be a wet blanket, but there is money to be made in the "autograph biz."
This all depends on what you have in mind for the photo. Is it a souvenir, or are you looking to do a little more? You've said that they're for you and that's terrific. Good for you.
There is a small, but growing market for auto'ed bonsai stuff. It's not a big deal, but it's there. Signed copies of Bonsai Techniques go for more than unsigned. I have a book signed by Saburo Kato, John Naka and Kimura that I bought a few years back for more than what the regular book would bring. Thought it would be kind of "cool" to have the mojo of those signatures.
This kind of thing is hard to pin down and can wind up (in larger markets and hobbies) hurting some people. What was once a courtesy by a sports star, artist, or whoever, has become a for-profit service with a back market that can turn alot of the people signing stuff off. For instance, professional skateboader Tony Hawk hesitates to sign skateboard decks for kids because the minute after he signs, some show up on Ebay for triple the price of an unsigned board. He wised up that he was not only being taken advantage of, he was losing money ;-)
I hardly think that kind of thing this is about to happen with bonsai, but you are relying on the goodwill of artists to do a favor for you out of their love of the hobby. Don't wear out your welcome...