I suspect that even if photographs were copied for use in questions and/or answers where, it would probably qualify as "fair use" at least under US law. The law (17 USC, §107) says:
[...] the fair use of a copyrighted work, [ ... ] for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include —
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.
At least IMO, the pictures that prompted this question were included for the purpose of comment, teaching, or scholarship. Neither the OP nor anybody writing answers appears to have any intent to profit from the copied work. Though I haven't looked at the site from which these were obtained, it seems likely that they constitute a relatively small part of the whole. It seems extremely unlikely that the question or answers is likely to affect the potential market or value of the copyrighted work.
This all goes to reinforce the fact that even if a link were considered equivalent to a copy, the question of legality probably wouldn't enter the picture.
Personally, I think creating such links should generally be avoided, but my reasons are primarily pragmatic rather than being based on a rather poorly defined concept like "good citizenship." In particular, it's purely a matter of time before most such links go stale. That leads to one of two possibilities:
- The pictures weren't necessary to understand the question, in which case they probably shouldn't have been included at all.
- The pictures were necessary to understand the question, in which case the question and answers are probably all rendered (nearly) useless if/when the link(s) go stale (which almost inevitably happens eventually, and frequently happens quickly).