Natural sort is an Option in sort(1):

  for i in $(seq 2 10) ; do
    touch img_$i-hn.txt
  done

  ls img_* | sort -V
  img_2-hn.txt
  img_3-hn.txt
  img_4-hn.txt
  img_5-hn.txt
  img_6-hn.txt
  img_7-hn.txt
  img_8-hn.txt
  img_9-hn.txt
  img_10-hn.txt

And we have "sort -h" to sort the output of e.g. "du -sh *" properly.

Edit: formatting and add sort -h