Identify and assign most recent file to shell variable

There's a number of characters in file names that would make that fail. You can improve it with:

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

Leaving a variable unquoted in list context (in Bourne-like shells other than zsh) is the split+glob operator, you almost never want to do that. -- marks the end of the options so "$fn" will not be taken as an option if it starts with -.

That still fails if filenames contain newline characters, but not space, tab, star, question mark, right square bracket, or start with dash.

Best is to use zsh here:

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

(.om[1]) are glob qualifiers, they are a zsh specific feature. . restricts the glob to regular files (will not include symlinks, directories, devices...), om is to order on modification time, and [1] to only take the first file.

Note that if you want to assign that to a shell variable, that would have to be an array variable:

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

(not that it makes a lot of difference on how you use it later).

Listing file

You could reverse the logic on the ls a bit.

$ ls -t | head -n1

Details

   -t     sort by modification time, newest first

Now it shows up first so we can use head to return the first result.

NOTE: You could also sort the list by change time (ctime), though you're probably going to want to use modify time above - (mtime). The ctime is the last time the file status meta information was changed.

   -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

For example:

$ ls -tc | head -n1

Moving the file

To do the move more cleanly you'll want to wrap the filename in double quotes.

Example

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

This will work in the majority of cases, there are a handful of legal filenames where it won't, for example, files with new lines. But these, though legal, are rarely ever intentionally used.