I like to use sizimg in shell scripts to do batch processing. Here's one that I use to convert a series of images to another:

#!/bin/sh
if [ $# -lt 2 ]
then
echo Usage: $0 '<source family root> <destination family root> [options]'
exit
fi
srcRoot="$1"
dstRoot="$2"
shift 2
for file in ${srcRoot}*
do
suffix=`expr "${file}" : "${srcRoot}\(.*\)"`
echo sizimg "${file}" -o "${dstRoot}${suffix}" "$@"
sizimg "${file}" -o "${dstRoot}${suffix}" "$@"
done
exit 0