I get that you're wanting to have a feature added, but this behavior has never been in UBB.threads; UBB.threads is an internet discussion forum title, I have not ever seen embedding options like you're asking for in any competing products.
As the images have yet to be attributed to a post (as the MySQL last insert id command doesn't run until the post is created) the images are held in a temporary folder until the post is created.
Where in the future we may introduce other methods of embedding attachments to a post, the current method below is the suggested method:
- Go to the Full Editor page (either by creating a new topic or clicking the button in the Quick Reply box)
- Click the Attachment Manager link and add images to your post.
- Add a subject and the body of your post.
- Create your new topic/post.
- Copy the URLs to the images either in the attachment list or inline image display (note the "Max File Size for Displaying Uploaded Images Inline" option in the Control Panel at CP -> Display Settings -> General ->Topic & Post Page Settings)
- Edit the post and add your images using image tags.
- Submit the now edited post
