Otherwise they are in the public domain so that no one will possibly complain about how they're used.
You can use all of my uploaded material freely; even though I haven't licensed my own photos under public domain you're welcome to use them even for commercial purposes. Don't bother crediting me or emailing me asking "Can I use this or that photo for this or that?". You can. I mean, they're just photos, depicting public places, buildings etc. Anybody visiting the place with a camera can get the same photo. Needless to say, this is of course also true for the public domain-photos.