Dulac said:
That's not how it works. You need to ask permission and then cite the source unless they don't care if you do. They own the image. And I won't allow the image I published in this wiki to be on profit sites (seed venders for example), so I will dmca them if they don't cooperate.
In general images found on the web, unless specified by the creator, are fair game to be used so long as your give proper attribution (and you should, via a byline/caption) and it is not for commercial purposes. It is assumed that most upstanding people will ask permission, but for the most part there is a copyright notice (and creators should specify this IMO) for the image making asking permission unnecessary.
Furthermore, if you find an image on a page who's original source cannot be tracked down, who do you ask? They have to be dead for it to be considered in the public domain and how are you to know if they are dead if you don't know who they are?
Of course there are the unscrupulous types who go so far as to remove water marks and change the image to make it look like its theirs. There was a thread about an ebay seller that did this awhile back if memory serves -_-