#!/bin/bash
cd /home/pi/JPGS
fn=$(ls -rt1 | tail -1)
mv -f $fn /home/pi/WWW/webpic.jpg

#! /bin/sh -
cd /home/pi/JPGS || exit
fn=$(ls -t | head -n1)
mv -f -- "$fn" /home/pi/WWW/webpic.jpg

#! /bin/zsh -
mv -f /home/pi/JPGS/*.jpg(.om[1]) /home/pi/WWW/webpic.jpg

fn=(/home/pi/JPGS/*.jpg(.om[1]))

$ ls -t | head -n1

-t     sort by modification time, newest first

-c     with -lt: sort by, and show, ctime (time of last modification of
          file status information) with -l: show ctime and sort by name
          otherwise: sort by ctime, newest first

$ ls -tc | head -n1

$ mv -f -- "$fn" /home/pi/WWW/webpic.jpg