Copy this into your .bash_profile! Bash function to batch-resize images according to the geometry specified in resize-arg. If no destination directory is specified, the converted images' filenames have "-resized" appended so as to not overwrite the original images.

batch_resize.sh

resize () {
    SUFFIX="-resized"
    if [ -n "$1" ] ; then
        if [ ! -f "${@: -1}" ] ; then
            SUFFIX=""
            DEST="${@: -1}"
            mkdir -p "$DEST"
        fi

        geom="$1"
        shift

        while [ -n "$1" ]
          do
            if [ -f "$1" ]; then
                FILENAME=$(basename "$1")
                EXTENSION="${FILENAME##*.}"
                FILENAME="${FILENAME%.*}"

                convert -filter Lanczos2 -resize "$geom" "$1" "${DEST}/${FILENAME}${SUFFIX}.${EXTENSION}"
            fi

            shift
        done
    else
        echo "Usage: resize resize-arg [file1, file2, ...] destinationDirectory"
        echo "For possible resize-arg formats, see:"
        echo "http://www.imagemagick.org/script/command-line-processing.php#geometry"
        echo ""
        echo 'If a destination directory is no specified, each filename will be appended with'
        echo '"-resized" so as to not overwrite the original files.'
    fi
}

Credit where credit is due: http://superuser.com/questions/282688/batch-resizing-images-in-os-x-to-a-certain-height/282909#282909