Imagemagick
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- create a copy resized inside a subfolder for every 48 64 128 256, of all png's in the directory
for n in 64 128 256 512;do [ -d $n ] || mkdir $n;for a in `ls -1 *.png *.jpg *.gif`;do convert -resize ${n}x${n} "${a}" "${n}/$a";done;done
