Value is always in terms of whatever an object 
means to someone, whether it's the current owner 
or a prospective buyer or whoever.

That being said, a lot of people buy cels because 
they like a particular character, or because they 
have an attachment to a particular scene. If 
someone's that kind of collector, ie, looking for 
a specific moment or a cel from a specific 
sequence, the cel is going to have much greater 
value in their minds because it's not just a 
picture, but it represents something greater... 
so-and-so's death scene, so-and-so's final 
showdown, so-and-so's moment of revelation, etc. 
Context is extremely important to the value of 
cels in some cases (an otherwise mediocre shot 
suddenly soars in value) and meaningless in other 
cases (the nice portrait shot might not be of 
anything key happening, but it's a good picture).

And that brings up one of the other main 
determiners of a cel's value... the quality of an 
image. If the lines are faded, or if it's a tiny 
shot that's way off-center, etc, context can only 
boost an image's value to a certain point. After 
that point, a prospective buyer might wonder if 
perhaps they might wait for a better shot from 
the same sequence to come along.

So, yes. If it's a great shot from a scene that 
has meaning to a collector of a certain series or 
character, it's going to certainly boost the 
perceived value to that audience. But there's 
always a limit (and it varies from person to 
person) as to how much context can compensate for 
other shortcomings if the actual item itself 
isn't ideal.