|Mrs. Tibitts (circa 1860)|
Longer answer (1) Once something's fallen in the public domain as a result of old age (as has all photography published in the U.S. before 1923), it cannot be reclaimed. If you modify a public domain work -- add additional imagery, unique coloration, or anything else arty -- you can claim your combination of old and new. But you will never be able to stop someone from reproducing the underlying public domain image with one possible exception. If you use a public domain image as your product's trademark, you may be able to stop competitors from using the same image on similar goods under trademark law.
Longer answer (2) It doesn't matter if someone "discovers" a public domain work and publishes it at their blog. If it's public domain, anyone can reproduce it. For more on the public domain, check out Steve Fishman's excellent book.