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)

-Cres