Via : https://hugogiraudel.com/2013/07/29/optimizing-with-bash/
adaptation sale, mais fait le boulot.
Pngoptimizer est disponible ici http://psydk.org/pngoptimizer

#!/bin/bash

PROGNAME=${0##*/}
INPUT=''
QUIET='0'
NOSTATS='0'
MOVE='0'
max_input_size=0
max_output_size=0

usage()
{
  cat <<EO
Usage: $PROGNAME [options]

Script to optimize JPG and PNG images in a directory.

Options:
EO
cat <<EO | column -s\& -t
    -h, --help         & shows this help
    -m, --move         & move files (deleting source)
    -q, --quiet        & disables output
    -i, --input [dir]  & specify input directory (current directory by default)
    -o, --output [dir] & specify output directory ("output" by default)
    -ns, --no-stats    & no stats at the end
EO
}

# $1: input image
# $2: output image 
optimize_image()
{
    input_file_size=$(stat -c%s "$1")
    max_input_size=$(expr $max_input_size + $input_file_size)

    if [ "${1##*.}" = "png" ]; then
        optipng -o1 -clobber -quiet $1 -out $2
        pngcrush -q -rem alla -reduce $1 $2 >/dev/null
    fi
    if [ "${1##*.}" = "jpg" -o "${1##*.}" = "jpeg" ]; then
        jpegtran -copy none -progressive $1 > $2
    fi

    if [ "$MOVE" == "1" ]; then
        rm -f $1
    fi

    output_file_size=$(stat -c%s "$2")
    max_output_size=$(expr $max_output_size + $output_file_size)
}

get_max_file_length()
{
    local maxlength=0

    IMAGES=$(find $INPUT -regextype posix-extended -regex '.*\.(jpg|jpeg|png)' | grep -v $OUTPUT)

    for CURRENT_IMAGE in $IMAGES; do
        filename=$(basename "$CURRENT_IMAGE")
        if [[ ${#filename} -gt $maxlength ]]; then
            maxlength=${#filename}
        fi
    done

    echo "$maxlength"   
}

main()
{
    # If $INPUT is empty, then we use current directory
    if [[ "$INPUT" == "" ]]; then
        INPUT=$(pwd)
    fi

    # If $OUTPUT is empty, then we use the directory "output" in the current directory
    if [[ "$OUTPUT" == "" ]]; then
        OUTPUT=$(pwd)/output
    fi

    # We create the output directory
    mkdir -p $OUTPUT

    # To avoid some troubles with filename with spaces, we store the current IFS (Internal File Separator)...
    SAVEIFS=$IFS
    # ...and we set a new one
    IFS=$(echo -en "\n\b")

    max_filelength=`get_max_file_length`
    pad=$(printf '%0.1s' "."{1..600})
    sDone=' [ DONE ]'
    linelength=$(expr $max_filelength + ${#sDone} + 5)

    # Search of all jpg/jpeg/png in $INPUT
    # We remove images from $OUTPUT if $OUTPUT is a subdirectory of $INPUT
    IMAGES=$(find $INPUT -regextype posix-extended -regex '.*\.(jpg|jpeg|png)' | grep -v $OUTPUT)

    if [ "$QUIET" == "0" ]; then
        echo --- Optimizing $INPUT ---
        echo
    fi
    for CURRENT_IMAGE in $IMAGES; do
        filename=$(basename $CURRENT_IMAGE)
        filepath=$(dirname "$CURRENT_IMAGE")
        if [ "$QUIET" == "0" ]; then
            printf '%s ' "$filename"
            printf '%*.*s' 0 $((linelength - ${#filename} - ${#sDone} )) "$pad"
        fi

        #optimize_image $CURRENT_IMAGE $OUTPUT/$filename
        mkdir -p ${filepath/$INPUT/"$OUTPUT/"}

        optimize_image $CURRENT_IMAGE ${CURRENT_IMAGE/$INPUT/"$OUTPUT/"}

        if [ "$QUIET" == "0" ]; then
            printf '%s\n' "$sDone"
        fi
    done

    # we restore the saved IFS
    IFS=$SAVEIFS

    if [ "$MOVE" == "1" ]; then
        find $INPUT -type d -empty -delete
    fi

    if [ "$NOSTATS" == "0" -a "$QUIET" == "0" ]; then
        echo
        echo "Input: " $(human_readable_filesize $max_input_size)
        echo "Output: " $(human_readable_filesize $max_output_size)
        space_saved=$(expr $max_input_size - $max_output_size)
        echo "Space save: " $(human_readable_filesize $space_saved)
    fi
}

human_readable_filesize()
{
echo -n $1 | awk 'function human(x) {
     s=" b  Kb Mb Gb Tb"
     while (x>=1024 && length(s)>1) 
           {x/=1024; s=substr(s,4)}
     s=substr(s,1,4)
     xf=(s==" b ")?"%5d   ":"%.2f"
     return sprintf( xf"%s", x, s)
  }
  {gsub(/^[0-9]+/, human($1)); print}'
}

SHORTOPTS="h,i:,o:,q,s,m"
LONGOPTS="help,input:,output:,quiet,no-stats,move"
ARGS=$(getopt -s bash --options $SHORTOPTS --longoptions $LONGOPTS --name $PROGNAME -- "$@")

eval set -- "$ARGS"
while true; do
    case $1 in
        -h|--help)
            usage
            exit 0
            ;;
        -i|--input)
            shift
            INPUT=$1
            ;;
        -o|--output)
            shift
            OUTPUT=$1
            ;;
        -q|--quiet)
            QUIET='1'
            ;;
        -s|--no-stats)
            NOSTATS='1'
            ;;
        -m|--move)
            MOVE='1'
            ;;
        --)
            shift
            break
            ;;
        *)
            shift
            break
            ;;
    esac
    shift
done

main