WRT this issue, there has recently been an interesting court decision in
Australia, while certainly not *legally* binding on Wikipedia, should
give some insight into the matter.
Basically, there is a panel talk show here in Australia called,
imaginatively enough, "The Panel", where a team of well-known comedians
sit around and ramble about the week's news events, chat with a guest or
two, and so on. In the process, they use short clips of vision from
other programs as a basis to comment on.
One of the other networks sued and lost, on the basis that in Australia,
copyright of a "cinematograph film" or "TV broadcast" only extends to
a
"substantial part" of that film or broadcast. They appealed this all
the way up to the High Court and lost, on a split decision. The
(probably quite strong) fair dealing defence was never tested, in the
end. The dissenting High Court judges thought that the majority was
deliberately misinterpreting the law to achieve a victory for common
sense.
More generally, it seems to me that there is a tendancy round here to
assume laws are written the same way as programming language definitions.
From my layperson's perspective, that seems to be
quite wrong - there's a
lot of fudging and playing around with ambiguities to
get the "right"
outcome, and screw the pedantry.
So my gut feeling is that the common sense aspect of using film stills to
illustrate an encyclopedia article makes it very, very unlikely we'll
ever get challenged on it, and that if we ever were we'd be highly
likely to win any court case.
Of course, I could be completely wrong :)