What is the difference between plagiarism and copyright infringement?
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).