Hey Bob,
First of all, images are "dumbed down" for posting here--way down. Someone might be able to steal them and post them elsewhere on the net, but I doubt very much they could gain any commercial value from them. (That's hard enough for me to do with the originals and the right connections.)
I don't mean to say it doesn't happen, because it happens to me fairly often. I occasionally find my articles, and less often my pictures, posted on obscure websites. They're not stealing them directly from me, but from print publishers who post my articles on their websites to attract subscribers to the print publications. Thieves aren't very smart, and most of them don't even take my byline or cutline credits off, and I find them easily by googling my name. (Putting watermarks or your name on images, by the way, doesn't slow thieves down in the least. They might even see it as giving the image professional credibility and more theft value.)
Now, just for the fun of it, I have occasionally posted threatening messages on these sites, but even though it technically is an infringement upon copyright laws, prosecuting such petty operators would be much more costly than anything I could gain. Finally, I decided to accept it as a form of flattery. If a major company with a bunch of money did it, that would be different, but they wouldn’t think of doing something like that. The ones who do it are idiots who most of the time don’t have enough money to even keep up their websites for very long.
Most of what I sell is for “one-time” rights, so it doesn’t matter where it has been published, or how many times. (Most of the articles and accompanying pictures I produce each week are sold to four to eight different publications at the same time, plus to my newspaper syndicate, which now is down to just four papers. Times are tough.) Besides, even though posting something here is technically “publishing” it, print publishers with thousands in circulation don’t consider one of the multi-millions of websites with relatively limited exposure as “published.” Not really.
Nevertheless, what I don’t post here are images I think have cover possibilities, unless they already have been published on a cover, or images I plan to sell to publishers who buy “first rights” or “first North American serial rights.” (I’m not sure posting those images here would really make any difference to them, but I’m playing it safe.)
I quit selling “all rights” to anything years ago when I determined I was in this for the long haul.
I’ve found a comfortable niche in smaller markets and actually make more selling something small a half-dozen times (to publication that know me and publish almost every thing I produce) than dealing with the demands, competition and endless frustration of the better paying top markets.