for year in 20??; do 
    time ( 
    for x in $year/in/**/*.jpg $year/in/**/*.JPG; do 
        if [[ ! -e "$dest_folder/$x" ]] || [[ "$x" -nt "$dest_folder/$x" ]] ; then
            nice convert -quiet -quality $quality -resize $resolution $x $dest_folder/$x
        else
            echo Skipping $x
        fi
    done
    ) &
done