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?