Recursively go through directory and make *.png *.jpg file gradient transparent(half to bottom). ImageMagick needed.

gradientTransparent.sh

#!/bin/bash
#caution!! for file name with spaces may fail
for f in *.png *.jpg
do
    size=`convert $f -ping -format "%wx%h" info:`
    echo $size >> tmp
    mkdir -p "$size" && cp $f "$size"
done
cat tmp | sort -u > index.log
echo "category done"

while read dir
do
    mkdir -p gradients && convert -size $dir gradient: -sigmoidal-contrast 5,10% gradients/$dir.png
done < index.log
echo "touch gradients done."

while read dir
do
    cd $dir
    for j in `ls`
    do
        mkdir -p ../trans && convert $j ../gradients/$dir.png -alpha Off -compose CopyOpacity -gravity South -composite ../trans/$j
    done
    cd ..
done < index.log
echo "compose transparent done."