I believe they do that, because no arrest has been made. And where the only battle right now seems to be in the court of public opinion, and where it's possible that this child's size had to do with the prejudice that killed him, perhaps his child photos are the wisest to produce.
My thing is... make the arrest. Because all of this time is just causing people to polarize this issue, which is just going to cause an un-needed division between people. At the very least, there is substantial evidence for this case to see the courts.
This was an innocent boy with Skittles and an Iced Tea (no weapons, no intention of committing wrong), and he was walking home from a 711. He was on the phone with his girlfriend during the stalking of Zimmerman (which phone records indicate), in which she had heard Martin ask what the guy approaching him (Zimmerman) wanted (after Zimmerman was told by dispatch not to follow). A neighbor also reported screaming for help by a boy during that struggle, and that boy was not 6'3, he was 6' (not that it matters in the killing), and was outweighed 2/1 by the aggressor (who was also armed, and could have kept a safe distance). He noted before approaching the boy (to dispatch) that the boy was running, saying "they always get away". And where this child was just trying to get home (after being stalked by a suspicious car (that was not a cop), it was probably smart to run and "get away". For all he knew (up until the second he died), he was being chased by a dangerous man (and he was). I think we would teach our children to run from people (who aren't cops) that pull alongside you and follow you at night, because I think every adult would have that same instinct.
The longer that this drags out by people saying "it's not about race", the longer this case becomes about race. I'm not even stating my opinions of whether it was race... personally, I feel like Zimmerman was just an overly paranoid and vigilant night-watch, in which race was just an aspect of his paranoia (not to be compared to most people). His own neighbors had complained to police about his over-aggression, and he was arrested in 05 for fighting with police.
This case just needs to go to court. It's not fair for the family to have their child be minimized in such a way.