I think you have to make a distinction between two things you mention: the image and the process whereby you arrive at the image. Replicating someone's image and taking credit for it is the most unambiguous type of plagiarism. Case closed.
With process, i.e. Ansel's explanations of his photos, or Les McLeans's, etc. The problem is different. If someone showed you how to achieve a warmer tone in your picture, and your next portfolio does, then you've just acquired knowledge. But if you go further, and you try to replicate one's approach towards light, composition and materials, then the line between learning/imitation and plagiarism could be finer.
There are plenty of photographic "tricks" that have been reused over and over: zooming while exposing, for instance, using a macro flash ring, etc. Some of these "tricks" are specific to certain photographers, but it would be foolish to copyright them. At the beginning, right after someone discovered such a trick, it's clear that the next photographer in line to reproduce it is probably directly copying the first photog's approach. Eventually everybody uses it, and with enough variations that it becomes just another tool of the trade.
With computers we can go one step further: cf. the Ansel Adams filter computer program that came out. We are able to replicate a fundamental aspect of AA's photographic work. We're not just talking about an aspect of, but about the gestalt of his pictures, the look that makes one say "ha! it's AA." Yet even here, the stance taken by an artist towards this process should make a difference. If the picture is shown as the result of an original artistic quest, then it's plagiarism. If the picture is self-aware, or simply not claiming more than to be the result of a filter, it's not per se plagiarism for me.
In the end, it leaves us only with the image as the real site of debate. Composition, subject, recognizable items. For instance, the debate here on APUG that Ara had about his shot of a woman in a bathub with a big gun, vs the one his buddy took after he heard of it. I think there are only shades of gray here between plagiarism, borrowing, and genre. Plagiarism would have been to reproduce the most important details, borrowing is to reuse some motifs, and genre is simply sharing conventional motifs.
If you really want to debate plagiarism, in the end you have to limit yourself to what the law says, and if the law is not to your liking, you lobby your politicians to change it. My point is that plagiarism exists, is identifiable, and should be punished, but cannot be done so without taking a picture to trial. It's like the concept "obscene." You and I have probably a similar concept of obscenity, but what one of us would consider as such might differ considerably from what the other one would.