MLB has clear rules for this circumstance. If the interference by a ball boy is intentional, the ball is declared dead, and the umpire can do whatever thinks he needs to do to nullify the interference. If it is unintentional, the ball is still alive and in-play.
The comments in the official rules also explain what is meant by intentional vs. unintentional actions. It doesn't matter whether the ball boy was actually trying to interfere, it is whether he was acting intentionally in what he was doing. So if he is in the wrong spot at the wrong time and gets hit by the ball or the player, that is unintentional.
In the case of the example photo you posted, he is trying to catch the ball, and that would be considered intentional. In that game, it actually turned out that the ball boy missed the ball, the player did not collide with the ball boy, and the umpire did not call interference.
From the MLB Official Baseball Rules, 2013 Edition, Rule 3.15:
3.15 No person shall be allowed on the playing field during a game except players and coaches in uniform, managers, news photographers
authorized by the home team, umpires, officers of the law in uniform
and watchmen or other employees of the home club. In case of
unintentional interference with play by any person herein authorized
to be on the playing field (except members of the team at bat who are
participating in the game, or a base coach, any of whom interfere with
a fielder attempting to field a batted or thrown ball; or an umpire)
the ball is alive and in play. If the interference is intentional, the
ball 28 Rule 3.10 to 3.15 shall be dead at the moment of the
interference and the umpire shall impose such penalties as in his
opinion will nullify the act of interference.
Rule 3.15 Comment: For
interference with a fielder attempting to field a batted or thrown
ball by members of the team at bat or base coaches, who are excepted
in Rule 3.15, see Rule 7.11. See also Rules 5.09(b), 5.09(f) and
6.08(d), which cover interference by an umpire, and Rule 7.08(b), which covers interference by a runner.
The question of intentional or
unintentional interference shall be decided on the basis of the
person’s action. For example: a bat boy, ball attendant, policeman,
etc., who tries to avoid being touched by a thrown or batted ball but
still is touched by the ball would be involved in unintentional
interference. If, however, he kicks the ball or picks it up or pushes
it, that is considered intentional interference, regardless of what
his thought may have been.
PLAY: Batter hits ball to shortstop, who
fields ball but throws wild past first baseman. The coach at first
base, to avoid being hit by the ball, falls to the ground and the
first baseman on his way to retrieve the wild thrown ball, runs into
the coach. The batter-runner finally ends up on third base. Whether
the umpire should call interference on the part of the coach is up to
the judgment of the umpire and if the umpire felt that the coach did
all he could to avoid interfering with the play, no inter-ference need
be called. If, in the judgment of the umpire, the coach was attempting
to make it appear that he was trying not to interfere, the umpire
should rule interference.