Looking at the topmost answer of this question, I wondered if the inclusion of that Dilbert comic might be a copyright violation, and what might be the proper way to handle it. More general: how should we act when we see a posting which includes material which might be copyrighted? Edit out the material ASAP and inform the author about the edit?
I don't think anything needs to be done.
Those of us with diamonds are not required to do anything - there's a DMCA process in place for copyright holders to use to contact the employee team and have action taken and it doesn't involve moderators.
Before you do anything, though, consider fair use. It's OK to include copyrighted material for some purposes, and it does maintain attribution (there's a clear statement of the owner and copyright on the image itself). If it was unclear as to the ownership (the poster passed it off as his own work), there may be more of an issue.
If the material can be replaced with an equivalent
Do it. It may take a little bit of work to create an equivalent drawing or graph, but if it's out there - do it. An example of this being done can be found on SO with a graph of big O growth were an image that was copyrighted was replaced with one that was CC licensed.
If the material is irrelevant
Remove it. Someone pasting an image of a croissant as something that was purely cosmetic and had no relevancy to the actual problem - delete it. It will probably make for a better question too and allow people reading it to focus more on the textual material rather than an irrelevant image. This has nothing to do with copyrighted material - just that the material is irrelevant. It should be done for public domain, Wikimedia images, and anything else.
Check the licensing on it and make it comply
For example, xkcd is under a CC BY-NC license (non commercial). This is further elaborated that books, blogs, and newsletters are ok for reprinting individual comics and we likely fall into that category. Though we need to link back to the original material.
Consider fair use
There is a lot of copyrighted material that we have on the site. Dilbert cartoons (as mentioned), images of book covers, excerpts from books, excerpts from papers, excerpts from documentation, blocks of third party code that doesn't have a compatible license with CC BY-SA, and the list goes on.
Most of these probably fall under fair use. If one sees something that is copyrighted and doesn't identify its source and the license that the source is under, it should probably get fixed.
The relevant bit from CC FAQ (under CC BY-SA 4.0 - see what I did there?) is:
May I apply a CC license to my work if it incorporates material used under fair use or another exception or limitation to copyright?
Yes, but it is important to prominently mark any third party material you incorporate into your work so reusers do not think the CC license applies to that material. The CC license only applies to the rights you have in the work. For example, if your CC-licensed slide deck includes a Flickr image you are using pursuant to fair use, make sure to identify that image as not being subject to the CC license. For more information about incorporating work owned by others, see our page about marking third party content. Read more considerations for licensors here.
If you feel that it is an egregious violation that you can't handle with the tools available...
Notify the copyright holder. Provide them the appropriate information for issuing a DMCA takedown of the material. Do note that unless you are the copyright holder, you cannot submit that material - only the copyright holder has the ability to do so (it gets into slander of tile and the like - you don't know, maybe the copyright holder did license the material - ask about that on Law.SE).
This is all about being a good janitor
The key takeaway from all of this is that one should be a janitor and curator of the material whenever possible and time permits (we are all volunteers afterall). There are ways that we can use copyrighted material when presented correctly. It is our job (yes, volunteer) to make the site the best it can be and that involves the occasional legwork of fixing up old posts so that the material in it becomes more useful - and that in part means making sure that we are as good netizens as possible with respect to the copyrights that are out there.