How Can I Stop Colorizers From "Stealing" My Antique Photo Collection?

Dear Rich: I have an extensive collection of original antique photo prints. Some are art photos, some are amateur/snapshot photos, some are news photos from Acme and other agencies. I foolishly posted a few on Pinterest a long time ago without watermarks. There are colorizers who swipe images without permission, colorize them, then present the result as their copyrighted art. Instagram has stopped removing colorizations of my photos when I report them saying I can't prove I hold the copyright to the original. I realize the news photos are copyrighted by Getty most likely (they charge $100 per hour to find out), but what about the art photos and private amateur photos? How do I establish copyright to protect my collection?
Owning a photographic print may have value as a vintage collectible but in order for you to stop colorizers from copying and modifying your collection, you need to acquire copyright in the photos. That's unlikely for two reasons: (1) most of the photos are likely in the public domain in which case, no one can claim copyright, and (2) if copyright still existed for some of the photos, you would need to track down the owner (the photographer or the person or company who acquired it from the photographer) and buy the rights. That's a considerable expenditure of time and money.
If the photos are public domain, how can Getty claim copyright? As the Library of Congress explains (in regard to Acme photos), news photos published before 1963 are likely public domain because they weren't renewed. Getty cannot assert copyright in public domain pictures but they may be betting that users would rather pay the licensing fee versus proving that copyright has expired. In short, users are paying to access high-quality scans of vintage photos.
If the photos are public domain, how can colorizers claim copyright?
As we explained a few years ago, it is possible to register a colorized black and white photo with the Copyright Office provided that the results "reveal a certain minimum amount of individual creative human authorship." Analogizing as to how the Copyright Office processes colorized motion pictures, the colorizer would have to demonstrate (1) numerous color selections made from an extensive color inventory; (2) a range of additional colors that is more than a trivial variation, and (3) that the overall appearance of the picture must be modified. A colorized image is considered a derivative work and the copyright extends only to the color choices. If the underlying work is not in the public domain, the colorizer will need permission from the copyright owner to reproduce or sell the derivative (or risk a charge of infringement).