Depends mostly on state privacy laws, unless you are also implicating trademark issues.
Most jurisdictions give right of privacy... a right against intrusion into one's physical solitude. Unless you have permission, you should not post photos that show anything private about a person, or anything that might be embarrassing. Avoid pictures of children. Avoid also anything that might look like you are trying to profit or benefit off of the post, as most states recognize the right to publicity belongs to the person you took a photo of, and not to you. Avoid also posting photo of a celebrity, if posting it can appear as being done for your own commercial gain.
And I'm assuming you took the photo, not someone else, as the copyright to a photo belongs to the one who took it.
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As my colleague suggests, little expectation of privacy in this public setting. Of course, the other cautions apply, such as celeb, etc. Facebook is populated by billions of snapshots, but you can still avoid trouble by applying common sense as to someone else's privacy. I can see nothing good coming from posting someone else's picture without their permission.
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Your basic right is actually pretty simple: if you're in a public place and you can see it, you can shoot it. This means as long as you're in a public location you can legally take almost any picture. However, if you're using a telephoto lens, parabolic microphone, or hidden camera to get a shot of a private property when you're standing on public property you might have an issue if someone on that property has an expectation of privacy. So, what constitutes a public place? Most places are obvious, a park, a street, a soccer field—these are unquestionably legal places to take pictures of anything happening. But what about all those Instagram photos of food you've taken inside a business? That's a little different.
Generally if a private property is open to the public (like a restaurant, retail store, tourist areas, etc) you are allowed to take photographs and video unless it is expressly posted somewhere on the premise that you can't. In most cases it's okay to assume you're allowed to take pictures and video in a shop that doesn't expressly forbid it. However, if a property owner (or store employee) tells you to stop, you have to stop. More importantly, use good judgement and assess the situation and environment before snapping pictures.
This also goes for citizen journalism. If you see an accident you want to record, public servant misconduct, or even TSA checkpoints, you can do so as long as you're not interfering with police or medical operations. As far as the Department of Justice is concerned you're also allowed to shoot video or still shots of police officers provided they're on public land. Videotaping police officers is still a tricky situation without a concrete ruling, but the courts have leaned toward protecting your right to film officers.
As with most laws you'll find some exceptions to the rules. Photographing on any clearly marked private property is considered trespassing. As for public government property you're mostly okay, however you cannot take photos of most military bases or inside most courthouses. A few other big caveats exist as well.
Just because some places are public doesn't make them legal for photography. For instance, a bathroom is a public place, but people have an expectation of privacy in the bathroom, so photos are typically not a good idea. This is also the case with anywhere else people might expect privacy, including inside places like AA meetings or doctor's offices.
The same goes for photos of people in a private space where they have an expectation of privacy, even if you're on public property. So, if you can see in your neighbor's window from the sidewalk while they're showering, you can't take that picture, even though you're on public property (and you might want to tell your neighbor to close their curtains). The general rule is basically if you didn't want someone covertly taking a picture of you in a semi-private place, it's probably not a good idea to take your own picture. These rules may vary from state to state, so check local laws before you're labeled a "peeping tom." If you do get caught taking a photo you shouldn't or if you're accused of taking taking an illegal picture when you're in the clear your response should be about the same.
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