Thanks for your help, I'm not sure if it was an issue with the extension, or a conflict between my template (Joomla Bamboo) or my JCE Editor but I found my own solution.
The Facebook debug tool indicated that my code was also not providing the correct opengraph tags, ie: og:image. Therefore Facebook could not determine which image to use in the shared post so it selected one at random.
I discovered a post, www.dart-creations.com/joomla/joomla-tut...cards-to-joomla.html
, that showed me how I could created a template override to add the correct meta properties for an og:image and with some editing and trial and error, I stripped the strings relating to Twitter and together with the Facebook debug tool, developers.facebook.com/tools/debug/og/object/,
I now have the correct images posting to Facebook. Please note that it was necessary to debug each article URL in the Facebook debugger to make it work.
I hope this helps anyone else if they experience a similar issue.