When you register your copyright, you must indicate whether the photographs you are registering are published or unpublished. The only time you are permitted to use a single registration to register a collection of multiple photographs (which is cheaper and easier), is when you are registering either a group of unpublished photographs, or a group of published photographs that was published in the same calendar year. There are many situations where a photographer may have posted some photos online in a piecemeal fashion, but now wants to publish them in a more professional context, and register the copyright. The question is, are the posted photos already “published”?
Neither copyright law nor the Copyright Office answers this question directly, and relevant case law is limited. The definition of publication in copyright law is as follows:
“Publication” 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.
To perform or display a work “publicly” means—
(1) to perform or display it at a place open to the public or at any place where a substantial number of persons outside of a normal circle of a family and its social acquaintances is gathered; or
(2) to transmit or otherwise communicate a performance or display of the work to a place specified by clause (1) or to the public, by means of any device or process, whether the members of the public capable of receiving the performance or display receive it in the same place or in separate places and at the same time or at different times. 
Based on this definition and relevant court cases, the American Society of Media Photographers recommends following the guideline that if the photograph was posted on a blog or website that was available to the general public, or on social media sites, it should be considered published, whereas if it was only posted on a password-protected section of a website, or a print was displayed in a gallery show, it may be considered unpublished.