What is the difference between plagiarism and copyright infringement? In this several times up-voted answer, it is suggested, among other things, that 'if someone using an image [...] that they do not own (it) is inappropriate and should be first reported to the PI of the paper and, potentially, the publisher if no action is taken.'

In my understanding, using images you do not own is not a good idea, it's illegal in many countries and the owners of the copyrights might react and claim their rights, but it's not plagiarism per se.

Let's consider the authors of an image processing paper who use a copyright-protected stock photograph to test their algorithm. They can cite the source of the image, in which case they would still be infringing copyright. But let's say they don't: they are not claiming that the photograph is their own, they just figure the readers won't care. 

Edit: I recently came across a paper where it was written that images were from a commercially available CD of example images, without saying which one. In this case it's clear that they do not claim that they generated the images themselves but they didn't give any reference.

Is this academic misconduct that should be reported?
To the best of my understanding the two differ in the following way:


Plagiarism is primarily an ethical issue: it refers to a false claim of creative work.
Copyright is primarily a legal issue: is refers to use of a work without a legal right to do so.


They can be confusing to differentiate because a person committing one is also often committing the other as well.  However, it is possible to violate copyright without plagiarizing and to plagiarize without violating copyright.  For example:


Darwin's "Origin of Species" text is old enough that it has entered the public domain, and thus is no longer protected by copyright.  A person who claimed chunks of it as their own would be plagiarizing, but not violating copyright.
If a person reproduces an image in a new paper with appropriate citation to its original but fails to pay the publisher of the original paper a $35 fee that publisher demands, then they have not plagiarized, but are in violation of copyright.


From a scientific perspective, plagiarism is a major problem, since it is a deliberate ethical violation that significantly undermines the credibility of the author.  Copyright violations, on their own, are much less of a big deal, since they may well be caused by legitimate misunderstanding or disagreement about the interpretation of a minor unclear point in a gigantic wall of legalese.

Thus, in the example given of image processing being applied to an unattributed image: if the contents of the image are not of scientific significance, I would interpret it as primarily an issue of copyright and thus not a significant violation worth reporting (as a scientist).