Is it illegal to take pictures of random people in public and post on twitter?

[Yeti]

Depends on detail and why you're doing it. There can usually be some limitations, such as if people still had a reasonable expectation of privacy, or if you're presenting them in a defamatory manner. Sometimes people can complain you're interfering with their public image when you're posting them publicly like that, too. You're safest if they're "art" shots and you're handling them as such... but you wouldn't normally be posting such things online, either, if that's truly what you're up to. The no-no you'd get into is if you use them commercially, such as to promote other products rather than the images themselves.