sample script that loops through all users and sets a different user picture.

userpictures.sh

#!/bin/bash

# create an array from all images in the User Pictures subfolders

IFS=$'\n' read -rd '' -a pictures <<< "$(find '/Library/User Pictures/Fun' -name *.tif -print )"

# loop through all users

picIndex=0

userList=$(dscl . list /Users UniqueID | awk '$2 > 500 {print $1}')
for user in $userList; do
  echo "$user"
  # delete JPEGPhoto
  dscl . delete "/Users/$user" JPEGPhoto
  
  # update path in picture
  echo ${pictures[$picindex]}
  dscl . create "/Users/$user" Picture "${pictures[$picindex]}"
  
  ((picindex++))
  if [[ $picindex -eq ${#pictures[@]} ]]; then
    picindex=0
  fi
done