As for myself, I see nothing wrong with altering 
a scanned image to remove things like dust--to 
me, that is only trying to create a more 
accurate image of the cel, not trying to alter 
it into something else.  Also, if it's for only 
display purposes, I feel like an owner can do 
what ever they want to the image-  That is NOT 
how I feel if the owner is selling the piece, 
however.  If trading or selling an item, I think 
it is important for the seller to be as open and 
honest as possible, and that includes accurately 
displaying and describing a cel.  If the image 
is altered, I would expect that the changes 
would at least be noted....and that's my two 
cents..:)