Okay, so, I see a lot of problems with people getting works stolen off the internet because the art is in a “public space.” While yes, that is true, the internet is public space and art is posted there, but that doesn’t make it public property, properly known as “Public Domain.”
“Art becomes public domain only after 70 years after the artist who made the work has DIED, 95 years after publication, or 120 years after creation, which ever comes first”. (Source). This is why you are allowed to use famous works like Van Gogh’s “Starry Knight” or Da Vinci’s “Mona Lisa” freely with out copyright issues, the artist have died well over 70 years ago, thus the work they have produced is now free for the pubic to use.
Posting art on the internet is not considered published works. “Published works are works that are made into copies able to distribute to the public.” (Source). Posting art on a site like tumblr, deviantart, message boards, youtube, vimeo, or any other number of websites are meant for display, not for redistribution unless the artist as stated that: “This work is available for redistribution.” However, if you do not see that sentence anywhere, then that art is purely for display only and you must ask that artist permission to use that work unless you are favoriting it on deviantart, reblogging/liking on tumblr/blogspot, adding the video/song to a playlist on youtube, etc.
Now, until the art work is under public domain, the works in question are protected under “Intellectual Property.” Where artist, inventors, writers, creators in general, are protected under the constitution and have full ownership of the works they have created from the moment it is created. (Source). Taking someone’s work that does not belong to you is considered theft and punishable by the full extent of the law. Meaning, the case can be taken to court and serious repercussions will ensue for the offender. (Source). The works are not just protected in the US, but world wide, and outlined in “article 27 of the Universal Declaration of Human Rights.” (Source).
(1) Everyone has the right freely to participate in the cultural life of the community, to enjoy the arts and to share in scientific advancement and its benefits. (2) Everyone has the right to the protection of the moral and material interests resulting from any scientific, literary or artistic production of which he is the author. (Source).
TL;DR: Yes the internet is public space, but that does not mean the works on the internet is considered public domain. All works are protected by Intellectual Property until it has been 70 years after artist’s death, 95 years after publication, or 120 years after date of creation; whatever comes first. Posting works on the internet does not count as publishing.
It is safe to say, any works created now will not be free to use publicly in your lifetime unless stated otherwise by the creator them self.