Here you go, look at the arrows I put on your 
scan. The bright pink arrows point to lines that 
should be on the *top side* of the cel - you'll 
need to keep the alcohol away from them! Mostly 
these will be all the lines that are colored and 
not black. The lines that I've pointed at with 
the purple arrows are the "black lines" (even if 
they're a tad faded) and may or may not be on the 
surface of the cel. I know that I have a cel of 
Sailor Pluto (didn't the same studio do both 
Sailor Moon and Dragonball??) where there were 
some of the black lines on the topside of the cel 
and some on the back side under the paint. It was 
weird. o_0  Since there's no way to tell without 
damaging it, I would stay away from them to be on 
the safe side.

If you have any doubts at all about the mark 
being a stain that you might not be able to 
remove, I would try the alcohol trick at the spot 
I pointed to with the green arrow. You have the 
most room there to give it a bit of a scrub 
without as much risk to the lines. if you find it 
does come off, one way you can get between the 
lines in the finer spots is to pull off most of 
the end of the Q-tip. When all you have left is a 
tiny wisp, wrap that wisp around the Q-tip stick 
and voila - you have a finepoint Q-tip. ^_^  But 
the alcohol will dry up fast when the Q-tip end 
is smaller is the only problem. . .

Many Sharp Smiles,
--Drac