There are limits regarding the right to take pictures in public. Instances of creepshot photographers have raised issues of good faith. For-purpose media (a street scene in the news, for instance, requires that any foreground person must have consent, or must be censored out.
So, dependjng on your state and county (or nation) it may be a crime to take pictures of someone else with an intent to use them as a foreground element without their consent (explicit or otherwise).