Copyright law draws a sharp, technical distinction between works that are “published” and works that are “unpublished.” So what is this distinction, why does it exist, and when do you need to care?
There are two big reasons why the law draws a distinction between published and unpublished works.
The first, broad reason is that one of the biggest rights of a copyright owner is the right to control the first publication of a work, in other words, the author of a work has the sole right to decide when, where, and how the work is released to the public. This is one of the strongest rights of a copyright owner and can deeply affect the outcome of a lawsuit - a good example is the famous case of Harper & Row v. Nation Enterprises, 471 U.S. 539 (1985). Former president Gerald Ford had written a tell-all memoir to be published with Harper & Row when tabloid newspaper The Nation obtained and published a juicy excerpt, and when Harper & Row sued, The Nation claimed that the publication was fair use, because President Ford was a public figure and the excerpt was therefore news. The Supreme Court decided against the tabloid, citing as a large part of its reasoning that The Nation had infringed President Ford’s exclusive right to first publication - because The Nation had published the excerpt before the full book had been released to the public. If Ford’s book had already been on the market - i.e. published - The Nation’s publication of the juicy excerpt would probably have been fair game, because Ford was such an important public and political figure. And whether a work is published or unpublished can affect damages in general - statutory copyright damages are only available for works that were registered within three months of their publication. The second, mundane reason is that works that are “published” must have a copy archived by the Library of Congress. A copyright is not some sort of natural property right arising from the application of labor to natural resources, unlike physical property, but a right granted by the government, and thus the government wants some record of all the rights it has given. The Copyright Office, which routinely publishes “circulars” which refine ambiguous areas of copyright law, has for a long time refused to say clearly whether a work posted on the internet - Instagram, Facebook, or Twitter, for example - is published or unpublished, and this author suspects that the mandatory archiving of all published works is a mundane reason why the Copyright Office has shied away from the question. The Library of Congress doesn’t want to become a print archive of the entire Internet.
The distinction between published and unpublished.
The Copyright Act states that “[p]ublication is the distribution of copies or phonorecords of a work to the public by sale or other transfer of ownership, or by rental, lease, or lending. The offering to distribute copies or phonorecords to a group of persons for purposes of further distribution, public performance, or public display constitutes publication. A public performance or display of a work does not of itself constitute publication.” 17 U.S.C. § 101. The Copyright Office adds that “[g]enerally, publication occurs on the date on which copies of the work are first made available to the public.” Here we see the real, hard distinction: “publishing” your work means allowing consumers to legally obtain copies of that work. Therefore merely publishing a photograph to Instagram is not “publishing” that photograph: consumers can see the photograph, and if they wanted they could screenshot it or otherwise make a copy, but such a screenshot would be, technically, copyright infringement. Any unauthorized copying is copyright infringement. To publish the photograph means to, somehow, make commercial use of it.
Now that can get very, very fuzzy. There are plenty of ways to “make commercial use” of a work without literally offering copies of it for sale. A one-of-a-kind painting hung in a gallery, for example, is “published,” because it has entered into the commercial stream. If you bought ad space on Facebook and used a photograph you took, that photograph would likely be “published,” despite the fact that you are not offering to sell copies of it - you have used the photograph for commercial purposes and are relying on copyright law to protect your exclusive use of it. If an installation artist never sold or made copies of their works but was occasionally hired to create works and posted photographs of their installations on Instagram, then those posts might well be considered “published” for purposes of damages, because they exist to bolster and build the artist’s commercial reputation and profile.
Publishing and social media.
So how should you, an artist or other content creator, approach posting your works on social media and navigating the published/unpublished dichotomy? Your first takeaway should be that you register your works as soon as you can - as soon as you think a work is finished, register it, and avoid missing your chance for statutory damages in the event that somebody steals it. Never publish a work unless you have registered it first. Your second takeaway should be that if you are a professional content creator and you make your content available to consumers, even if you do not offer copies of that content for sale or lease, there is a chance that your content will be considered published, because that work is building your own artistic profile, and thus you will not be able to lean on your right of first publication in the event of a dispute. And your third takeaway should be that if you do post your works on social media, you should ensure that your works aren’t copied willynilly by marking them with a watermark or similar marking that states that the work is copyrighted and that any copies are unauthorized. If you make it clear that copies are not available, then you may have an easier time proving that the work is unpublished and thus may be able to lean more heavily on your right of first publication.
No matter where you are in the timeline of creating and publishing content, be aware of the legalities of your work and remember that you produce a product that is protected by law. Talk to an attorney and understand your rights.
 Or prior to the infringement.
 Fair use arguments could be made depending on how you use the picture you screenshot, but in general, any unauthorized copying of a copyrighted work is infringement.