#!/bin/sh

margin=100
aspect_ratio=177

extent=$((margin * aspect_ratio / 100))%x${margin}%
options="-gravity center -extent $extent"

for input
do
  case "$input" in
       *.*) output=${input%.*}.margins.${input##*.} ;;
       *)   output=$input.margins.png ;;
  esac
  convert "$input" $options "$output"
done