Testing...
OK, I've worked it out...
Attach the image as normal. Once back to the main window with the "reply" interface, scroll down to where the attachments are shown in a list, just above the "Manage Attachments" button.
If you click on an attachment, it will open in a new tab or window.
Look closely at the URL. Here's my one:
Code:
http://www.vintage-radio.net/forum/attachment.php?attachmentid=142089&stc=1&d=1493896996
The key thing is "attachmentid" - 142089 in this case.
If you quote this to reply, you'll see that I've enclosed that ID in the ATTACH tags. Job done
Perhaps there's a simpler way of doing it, but as I do a lot of HTML and similar stuff, I'm OK with this.
But the downside: it doesn't use the "Lightbox" plugin, so once you've viewed the image, click "Back" in the browser to go back to the post.
Hope this helps,
Mark