What is the best method to register when I have hundreds of unpublished images from over the years, but I have no idea when they were taken?
You may register those images individually by submitting each image with a separate application and filing fee. Alternatively, you may register all of the images as an “unpublished collection” by submitting them together with one application and one filing fee, provided that the following requirement have been met:
The elements are assembled in an orderly form;
The combined elements bear a single title identifying the collection as a whole;
The copyright claimant in all of the elements, and in the collection as a whole, is the same; and
All of the elements are by the same author, or, if they are by different authors, at least one of the authors has contributed copyrightable authorship to each element.
f the works in an unpublished collection were created over a period of thirteen months or more, the applicant should provide the year of creation/completion for the most recent work that will be submitted for registration. See Compendium (Third) §1106.3(D).
Rob Kasunic, Director of Registration Policy and Practices at the U.S. Copyright Office