When commenting on this post I found another good post that partly answered the question.
The related answer I found is - I believe - of good quality. However, it has an old screenshot that doesn't resemble the current WP interface.
If the answer is hard to understand with the old screenshot, then I'd say a replacement or an additional screenshot is justified. But if it just reminds the viewer how old the answer is, then I'd just leave it as is. It's a nice hint to review the solution carefully.
A new answer with just a new screenshot, as Howdy_McGee suggests, is probably not a good solution for that. An answer should have more weight, in my opinion.
As long as the meaning and intent/meaning of the answer remains the same, that the screenshot improves it in a tangible way, and it isn't substantially changed then I think it's a reasonable improvement, especially if it's your own answer being edited.
For example, updating a screenshot of a UI that has slightly changed, e.g. if a button has moved a little but it's still the same button on the same page sounds fine.
Likewise if an image wasn't uploaded to the site but links offsite and has been broken, this falls under fixing an answer that has suffered bitrot.
A bad example would be if the functionality has been moved or replaced, e.g. replacing a screenshot of the customizer with a screenshot of the site editor. That would be a bad change.
As Fuxia said though, old screenshots can sometimes imply that the answer may no longer apply. Verify that the answer makes sense and still stands on its own first. Screenshots can help support an answer but they shouldn't be the answer, much like links.
