#!/bin/bash

for f in *.DNG; do [ ! -f "${f%.DNG}.JPG" ] && echo "Will remove $f"; done
read -p "Proceed? " answer

case $answer in
    [Yy]* ) for f in *.DNG; do [ ! -f "${f%.DNG}.JPG" ] && rm "$f"; done; exit;;
    [Nn]* ) echo "Quitting.."; exit;;
esac