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."In transferring the data necessary to make an image show up on your screen, the receiving computer has to make its own copy--a reproduction rather than (or in addition to) a display.
I can't think of any form of technology that "transmits" information without copying to some degree, or without giving people an opportunity to copy it. Music over the airwaves gets transmitted to technology that can record and copy the info via itunes or mp3 play, almost all stations transmit via the internet as well. As for TV broadcasting, the information can also be copied and reproduced to accommodate satellite TV, tivos, and again the internet. Even displaying something in public in the presence of CCTV would mean that data is being copied, so I'm not really sold on that explanation.
As a former employee of the copyright office, I'll tell you that the Office takes no official position on whether something posted on the web is published (and employees are trained to say exactly that when asked by an applicant). They will leave it to an applicant to describe a work as published or unpublished, and leave it to the courts to sort it out.
As the guy who had the phone conversation with the copyright office person, I'll tell you that I was told was that "posting images on the net does not necessarily mean it was published, that it could be considered on display as unpublished work" That if the work was made available for sale or for distribution, that it would then be more likely that the work was published. They were very careful not to offer any advice or tell me what they felt my situation was, but they did offer that information as facts.