You mentioned

The download is perfectly fine. As I have mentioned, I can set the photo as wallpaper by right clicking the photo and chosing 'set as wallpaper' from the menu.

As you can manually set the wallpaper the problem may be on the cache, which is generated before the downloader completely write the data on the file. So regenerating the cache in this kind of case is necessary.

So delete wallpaper cache rm /home/$USER/.cache/wallpaper/* . Add this code to end of the script after some sleep.

Let's do a test

1. Find and open the image cache of current wallpaper from /home/$USER/.cache/wallpaper/
2. Edit the image ( such as rotate clockwise. You can do it with the default imageg viwer (GNOME image viewer) using Ctrl+R (Or from edit -> rotate))
3. Logout and login ubuntu (or restart)

That's it now you can see the changes on the desktop wallpaper (If you rotated the image, then the wallpaper will also get rotated)

Using bash for this task should be possible but I prefer doing complex array manipulations with a Python3 script:

#!/usr/bin/env python3

import argparse
import subprocess
import sys

parser = argparse.ArgumentParser()
parser.add_argument("schema", help="gsettings shema", metavar="SCHEMA")
parser.add_argument("key", help="gsettings key", metavar="KEY")
parser.add_argument("values",
                    help="gsettings value(s) to remove from the KEY array",
                    metavar="VALUE", nargs='*')
args = parser.parse_args()

array = eval(subprocess.check_output(["gsettings", "get", args.schema, args.key]))
for v in args.values:
    try:
        value = eval(v)
    except:
        value = v
    try:
        array.remove(value)
    except ValueError:
        print("Warning: {} not in {} {}".format(value, args.schema, args.key), file=sys.stderr)
subprocess.call(["gsettings", "set", args.schema, args.key, str(array)])

This script allow multiple values to be removed at the same time from the schema/key passed to the script.