Is there some particular 'totally neutral' photo combo available that everyone 'should' be using?
I don't watch TV, so I haven't personally observed which cable networks favor which photos. I know one reason why the "smiling tie" employee pic remained in relatively limited circulation (at least online) is because its source, the Orlando Sentinel, had acquired the pic, 3 weeks back, from an anonymous coworker of Zimmerman's. As such, they can't legally grant others permission to use it, and many outlets--not all, clearly--held back from 'borrowing' it on the off-chance of finding themselves in a fair-use lawsuit. Otherwise, until this week the only generally available photo of Zimmerman had been that "fat frowning" 2005 mugshot.
In Martin's case, his family made several photos available to media from the beginning. The most recent of those was the black-and-white 'hoodie pic,' but initially many outlets were reluctant to choose that one; I assume because he'd been profiled and, ultimately, shot while wearing a hoodie, so until the wave of 'Million Hoodie' marches normalized that image, it probably seemed like a macabre association. In late March his parents also made available several pics from a family party 9 days before his death, but I've seen very few outlets use those, perhaps because they have mutiple other people in them. The "grill-wearing," etc. pictures from Martin's Twitter were obviously taken without his family's permission, but it's Twitter, so, no permission required.
It's really not as easy as many assume for media to get their hands on fair-use pics of ordinary people who wind up in crime cases, and while there will always be journalists who do indisputably gratuitous things with photos, the reality is most photojournalists spend a good deal of time fretting over which pic to choose and why from the limited options available, whether it's OK to shift to new pics after other iconic images have become established and if so what's our justification for that, etc. I've not myself been much bothered by which outlets seem to favor which pictures in this case; the only thing that really perturbed me were some openly racist commenters, and in a few cases bloggers, who clearly felt that using ANY pictures of Trayvon Martin beyond (what they considered) the 'thuggish-looking' ones constitutes some sort of whitewash. But we're talking Stormfront-ish types there, so, par for the course. (And ironically, those same people usually have a similarly strong preference for "fat frowning" Zimmerman, because to them he looks more 'nonwhite' in that pic.)
Whenever I'm especially interested in some particular news story, I supplement my usual everyday sources (NYT, WSJ, LA Times etc.) by following the story on an automatic, algorithm-based aggregator like Google News or (for newsblogs) Memeorandum. That way I can skim through a much larger and more diverse array of sources in search of details and perspectives that, inevitably, won't be available just anywhere. Local sources, too--the Orlando Sentinel, Miami Herald, and Tampa Bay Times have in different ways all done an especially great job covering this case, IMO.