I needed to create drop shadows on around 200 images. I didn’t really like the idea of doing it with Gimp as that would have taken hours! It is really simple to do it using Imagemagick

convert
\( +clone -background black -shadow 80x3+5+5 \) +swap -background white -mosaic

I needed to convert a whole directory, so it’s just a simple case of wrapping it around a loop

for a in *.jpg
do
echo $a
convert "$a" \( +clone -background black -shadow 80x3+5+5 \) +swap -background white -mosaic "shadows/$a"
done