Under the Berne Convention for the Protection of Literary and Artistic Works, which is an international agreement, copyright is automatically held by the author or creator as soon as something—a text, a photograph, a video, a sound recording—is created, regardless of whether it’s accompanied by a (c) or registered with a copyright office. That copyright is valid for the life of the author + 70 years. That picture you want to repost from Flickr, for example? Unless it’s in the Commons, it’s under copyright — check the licensing. Open Access and Creative Commons licenses are changing the landscape of access and use, but they are not universal. When artistic and literary works are reposted on Tumblr, “oh I just found this somewhere on the Internet” or crediting to Pinterest are not sufficient attributions. It’s not only disrespectful to creators, it’s a violation of copyright law. I doubt anyone is going to sue a Tumblr user for reposting an image, but it’s important to be aware of copyright and grant creators their due.