Fair use is a slippery slope and incredibly difficult to pin down. That's what we have the courts for
Now I didn't see the thread on dpreview, but one could argue that it was "educational" and hence his use of your images were covered by fair use...especially if he receives no financial gain from their use. I'm not saying I agree with that, but it is a possible interpretation.
I have been at the edges of this battle for years, as an academic who ended up being a technology evangelist as well (although some days I feel more luddite). If you want to really get into the muck, start thinking about fair use for time-based media (ie video). My guidelines to my students were very clear - when you use any image that is not yours, you should include credits for the image. Forums are pretty informal beasts though, so it gets muddy and lax quickly. If one "steals" the image (from a server standpoint, ie imbeds the original image url in the post) then there is some degree of inherent "credit" - someone can look at the link to see the original source.
If one downloads the image then hosts it elsewhere, then they have removed the lineage and really should credit the original creator. BUT, the reality is that most people will post in a forum with no expectation of financial gain or receiving credit for an image. Photo forums are a little different because people often do "publish" their own work, and the assumption is that a photo in a thread was created by the author unless the context indicates otherwise.
In a perfect world the person would have credited your work in his post. We do not live in a perfect world. That is why for some of my images I watermark them. For other ones I don't...I feel somewhat pretentious doing it in certain venues and circumstances, but it is about the only recourse one has to try and maintain their IP with photos.
If people want to steal, they will. No matter what the technology is. All I can hope is that dharma is a b*tch...