From http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=270512
When averaging multiple images, it would be nice to have an option to
throw out grossly over- or underexposed pixels by simply excluding them
from the average. E.g. if pixel (x,y) is white in one out of 10 images
(and black in all others) it is likely that this single image simply has
a pixel defect. Being able to tell convert to "ignore color values that
are [treshold] brighter or darker than the standard deviation at this
pixel location" would then greatly improve the quality of the averaged
result.
I supose ignore-below and ignore-above option will work
Regards
Bastien