All pictures are copyrighted the moment they are taken in modern copyright law, they may never have a © attached to it but that doesnt make it any less copyrighted. As to not stating the URL and name of the site, this is a sure way to leave youself wide open for the breaking of a copyright.

Linking directly to the image is probably your best bet along with posting of the name of the site that posted the source picture you are using, assuming that the picture is a publicly posted image and not one requiring permission to view.

Of course there may be many pictures that the copyright holder has released into the public domain and getting permission to use it is not needed. However, since you do not know that for sure you can not count on that unless you contact the copyright holder to confirm it. Something that will prove to be difficult at best.

Bottomline is that unless you took the picture yourself and therefore are the copyright holder or you have received permission to post a copyrighted image by the copyright holder, you really should not be posting them at all.

Having said that, I think its safe to say no one will stop from downloading pictures, stories, or any other files they want and with the Internet being as it is I am sure they will have no problems continuing to use them in anyway they please whether they have been released into the public domain or not.