I tested to figure it out.
I also couldn't use an image URL from an image on the forum.
I copied the URL correctly , pasted it in Manage Attachments " Upload from a URL " .
I got this message " This is not a valid image file " .
What does work is to use the "Insert Image" button at the top of the Reply Window.
I had never posted an image from a URL on the forum . So glad to know how.
I usually just do a right click "Save Picture as" to my Pictures on my computer & upload from there.