Almost without exception, these images come from the vast store published under Creative Commons licences on Flickr, which is why an article just published in Wired gave me that sinking feeling. Flickr provides the flexibility for users to switch photos back and forth - without limitation - between CC licenses and "all rights reserved" copyrights. So all those images I've used, entirely legally, over the years could suddenly become illegal if the users decided to switch the licence terms. Except that they don't. CC licences are irrevocable, so I'm in the clear.
You can stop distributing your work under a Creative Commons license at any time you wish; but this will not withdraw any copies of your work that already exist under a Creative Commons license from circulation, be they verbatim copies, copies included in collective works and/or adaptations of your work.But how could I prove it? The only solution is to use a service such as the Internet Archive's Wayback Machine or WebCite to record Flickr page CC licence at the time of publication. (I hadn't come across WebCite before and it looks interesting, especially the bookmarklet.) But that's a heck of an overhead for each blog post I write, and I'm not sure it's achievable. My gut feeling is that the only practical solution for blog posts is to carry on as before and pull images if challenged, but that's probably not the solution for other types of online document.