Pretty much every Tumblr, Pinterest and other re-blogging site has, somewhere in the "about" section, some variant of the following text:
DISCLAIMER: All images, unless otherwise noted, were reblogged from the Internet and are assumed to be in the public domain. In the event that there is still a problem or error with copyrighted material, the copyright breach is unintentional and non-commercial and the material will be removed immediately upon presented proof.
This is bull, and everyone knows it.
Most photography and other art, in most countries, is copyrighted by default. Some jurisdictions may grant additional legal leverage if you formally register your copyrights, but – in the absence of specific licence terms stating otherwise – pretty much any image you come across online is the copyrighted property of its creator.
It is not easy to release something into the public domain, even if you are the creator and copyright holder. The Creative Commons CC0 licence is, generally speaking, the closest you can get; the other option is to wait 70 years or so until all the copyrights expire. "Assumed to be in the public domain" can safely be translated as "Almost certainly copyrighted, but I'll pretend I can cover my ass by saying this".
Whether these aggregation and re-blogging services are beneficial or detrimental to artists is a story for another day, or for the comments. Either way, let's avoid using home-brewed, clearly faulty legalese to pretend they're something they are not.