Copyright protection, under the U.S. Constitution, only lasts for “limited times.” That means eventually (at least theoretically) all copyrighted works, including photos that can be incorporated into a blog post, will enter the public domain at some point in the future.
So how old does a work have to be to enter the public domain?
Well, it’s complicated. You are generally safe assuming something is in the public domain if it was published in 1922 or before. (The hazy legal world of apparent exceptions are discussed here and here.)
Does that mean that next year works from 1923 will enter the public domain?
Unfortuntately, no. The reason why is that Congress has been, for decades now, regularly extending copyright terms at the bidding of the entertainment industry. The latest special-interest windfall was the Sonny Bono Copyright Term Extension Act (text, summary). Because of this most recent extension legislation, the public domain is stuck at 1922 and will be for quite a while.
The public domain won’t grow again because of copyright expiration until 2019 – unless Congress extends the copyright term again. And you can bet that special interests are lined up to lobby for that extension when the time comes.
Since Disney lobbied hard for its passage, the Sonny Bono Act was dubbed the “Mickey Mouse Protection Act” by some. Without the law, Mickey Mouse’s original cartoon short films would have entered the public domain by now.
But note that, because of sloppy work done in Disney’s early days, people who have looked at the matter closely (here and here) have concluded that the original Mickey Mouse and the motion picture Steamboat Willie (in which Mickey made one of his first appearances) are no longer the subject of a valid copyright. The public domain status of Mickey Mouse remains untested in court. People tend to be very afraid of Disney lawyers. I can’t imagine why.