I also admit to making two fairly drastic changes 
in scans.  In one case, I took a black rectangle 
and inverted it to white, to make the gallery 
image correspond to the actual use of the cel in 
the anime (where the black rectangle was also 
replaced by a shimmering white special effect).  

In the other, a cel of five Clow Cards, a thin 
strip of one card fell just off the left edge of 
the cel.  I realized that I could copy and paste 
a strip off the card next to the imperfect one, 
and so complete the image.

In both cases, the comments in the gallery from 
the original posting date have explicitly said 
that the image of the cel has been digitally 
altered and explained what the original cel looks 
like.  

I'm tempted, after reading this discussion, 
however, to go back to my original unretouched 
scans and post them up.  The consensus seems to 
be that a little dusting and mending is OK 
(provided that the cel is NFS) but that changes 
beyond this order are non-kosher, even when they 
are freely admitted.