The user embedded a picture as an attachment to a post in a thread they maintain as sort of a blog.
The files are uploaded to my server as actual attachments vs being linked from another site.
Today he wanted to post one of his pictures in another thread so he highlighted the photo and then "copied" it and then pasted it into a post in a different thread.
In the new thread the pictures did not display in the actual post, but instead show an icon for a picture with a red x on it.
If you reply you can actually see the pictures in the reply pop up but the disappear again after you finish your post quoting theirs.
Looking at the code view of the post editor, what you see is the URL for the attachment in the Xenforo system surrounded by img bbcode tags.
I'm going to attempt to give you an example here: