mass_rename_files_recursive.sh

#!/bin/bash

find . -type f -printf '%p\n' | while read file; do
  oldfile=$(basename "$file")
  newfile=$(echo "$oldfile" | sed 's/OLDNAME.jpg/NEWNAME.jpg/g')
  if [ ! "$newfile" == "$oldfile" ]; then
    mv "$file" "${file%$oldfile}$newfile"
  fi
done