Thank you @bizanimesh and @tamirat22,
Sorry for the delayed reply.
I had the following two codes in my .htaccess file.
<Files xmlrpc.php>
order deny,allow
deny from all
allow from 123.123.123.123
</Files>
RewriteEngine On
ErrorDocument 403 "Forbidden"
RewriteCond %{THE_REQUEST} "^POST /xmlrpc.php"
RewriteCond %{HTTP_USER_AGENT} "!^Jetpack by WordPress.com" [NC]
RewriteRule .* - [R=403,L]
However I have already tried deleting these codes but the results are the same. Many of the images were still broken.
After removing the codes I could successfully test my URL by putting xmlrpc.php at the end, and I could see the the following message:
XML?RPC server accepts POST requests only
But still many of the images were not loading and were broken.
I tried deleting the cache, still there was no improvement.
The problem is that regardless of whether the above codes are present or not in my .htaccess, some of the images load fine while some images do not load at all.
Strangely, the previous image link which I had submitted in the above comments is now loading correctly, even though I have placed the xmlrpc blocking codes back in my htaccess. You can check the image link bellow:
https://i0.wp.com/www.homemade-circuits.com/wp-content/uploads/2022/04/simple-LED-flashlight.jpg?resize=665%2C230&ssl=1
I have currently decided not to use jetpack boost plugin because the broken images affect user experience hugely.
Kind Regards
Swagatam