All taken before 1888 even if never published are in the public domain.
Published and taken before 1923. (note publication can be outside the US)
Published between 1923 and 1963 for which Time did not renew the copyright (unlikely)
All taken by authors who died before 1938 (however, Time doesn't identify the photographer that took the pictures which is unfortunate)
Additionally, I'm not sure that Google is using the term "unpublished" as defined in the Copyright Act of 1978. Published means part of a work offered for sale, or displayed with the intent to offer for sale even if never sold. For example, if the photographer sent the pictures to a group of persons (i.e. to multiple magazines) trying to sale them that would be publication (even though not offered to the public at large). Note that displayed with no intent to sale does NOT constitute publication.
Finally, my understanding is that if these pictures were incorporated into any television show that would be publication since the public was licensed to have a copy on their television during that television show, even though they may not have been authorized to keep a copy after the show itself.

On Wed, Nov 19, 2008 at 8:43 AM, the wub <thewub.wiki@googlemail.com> wrote:

11/19 Andrew Gray <shimgray@gmail.com>:

> 2008/11/19 David Gerard <dgerard@gmail.com>:
>> 2008/11/19 Michael Galpert | Aviary <michael@aviary.com>:
>>
>>> dont think that applies across the board since some of these images are
>>> being published for the first time
>>
>> Really? I thought the published/unpublished distinction didn't affect
>> that in US law.
>>
>> (I'm sure someone has full legal details to hand ...)
>
> It is indeed *published* before 1923, though we've been a bit vague
> about confirming this in the past. There's a very useful table of such
> things here:
>
> http://www.copyright.cornell.edu/public_domain/
>
> However, archives like these are still very useful - if it was made
> before 1923 for a news magazine, odds are high it were published quite
> promptly, and it's often a lot easier to take their image and confirm
> with a pre-1923 source than it is to ferret out the source and scan
> from scratch...
>
> --
> - Andrew Gray
> andrew.gray@dunelm.org.uk