Just because someone was trailing him, does NOT give him the right to turn around, approach the person following him, and attack. That makes him the aggressor, not someone defending himself.
There are plenty of other avenues to deal with the situation without resorting to aggression: Trayvon could have run (and easily out-run Zimmerman), could have yelled at Zimmerman "Stop following me!", could have knocked on someone's door for help, could have gotten 911 on his cell phone. Any of these options are preferable to engaging someone whose presence makes you fear for your safety.
It's simple: If Trayvon initiated the violent encounter (by attacking), then he was NOT defending himself. If Zimmerman initiated the violent encounter (by attacking), then he was NOT defending himself. Following someone (however ill-advised) does not make one an aggressor.
As for the source of the photo, it was apparently taken just a few minutes after the gunshots, by someone who saw the initial scuffle (don't know if he saw the shooting), and was approached by Zimmerman after the shooting, requesting that he call Zimmerman's wife. That's what I gathered from the news report here: http://gma.yahoo.com/warning-graphic-photo-possible-evidence-shows-george-zimmermans-050145810--abc-news-topstories.html
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