If you can pinpoint it to an episode, by all
means, it's nice to pass that information along
to the people who view your gallery. If you can
get a screencap of the frame, that's even better.
It gives a nice context-- it's not just a nice
picture to look at, but there's information
attached to it which makes it a little more
meaningful, and makes it stand out from all the
other cels of whoever out there.
I also like to scan in my sketches, when they're
interesting enough to warrant their own images
(and sometimes even when they're not ^_^), and
give random information, like how many layers
exist, if any of them are stuck, the cel number,
and so on.
Of course, giving an entire episode summary is a
little bit of overkill. You might just want to
explain the events leading up to that moment in
the show, or make some sort of comment about how
the cels were shot if it's noteworthy--- ie, with
one three-layer I have, "Saicho is now pleading
that Recca makes Kuukai's death an honorable one,
as there is little else left for Team Ku's future
at this point in time. I'm don't think this
sequence was ever shot with this combination of
cels in the same frame, but this is how it came."
In general, you come across as a little more
knowledgable about what you're collecting and
your visitors get a little extra context to
appreciate your collection. I doubt you can ever
ID everything, but the extra effort is always
nice and shows you care. :o)