I've just posted an answer that includes a link to an XKCD cartoon that is closely related to the topic. Is it appropriate to include such humor in an answer?
Absolutely it is.
As long as an answer still answers the question, linking to external resources, be they serious or comedic, is fine and actively encouraged.
This only becomes a problem when the humor is out of place or is the only thing in an answer, which is not the case here.
As long as the cartoon isn't the only component of the answer, it's fine. Pictures are encouraged to make up a supportive section of an answer. Having taken a look at your answer, it looks perfectly fine, you could always embed the picture :)
However, if an answer is only a picture, or only a link, or only a quote...
I hate to be a party pooper... but the answer as it was when I wrote this (see edit note) is infringed on XKCD's copyright.
You are required to:
- provide the name of the creator and attribution parties
- a copyright notice
- a license notice (name and link to the CC BY-NC 2.5 license)
- a disclaimer notice
- a link to the material
- the title of the material (I guess that would be "PGP")
Finally... all "user contributions" on stack exchange are licensed under the CC BY-SA license which is incompatible with the CC BY-NC license.
I'm not a lawyer but I think because of this last point it might be illegal (criminal in some countries with severe statutory damages) to include an XKCD comic in your answer even if you comply with the 6 things I listed above.
Thene answer including the comic can (and probably will) be used for commercial purposes. The XKCD's license prohibits releasing the comic under a license that allows commercial use.
This answer has some relevant details: https://opensource.stackexchange.com/a/1252/436
In summary, on topic humour is appropriate but not when it's licensed under terms that prohibit posting it here. You can probably only post a link to the comic page, rather than the image.
The answer in question originally contained a link to the comic, then somebody else edited it to include the image, and it has now been reverted back to having just the link.