If I'm not mistaken, the photos are copyrighted by their owners implicitly in US, Europe and many other countries. By using/distributing the photos without their permission, you are almost certainly violating their copyright and will open you up to lawsuits. And while it is legal to photograph people in public places, the commercial use of those photos (as opposed to editorial use) will require a signed model release.
If what you're doing is non-commercial, several Flickr photos are released under the Creative Commons License. Although the photographer may have released it that way, there's no guarantee the photographer got a proper model release from the subject of the photo.
In summary, if you didn't take it the photo (and occasionally, even if it is) and it's not explicitly stated as public domain, it would be illegal for you to use it commercially.