UT can and probably will answer better than I can. BUt my understanding is that it's just not set up to do that. But let me see if I can get around it in a very awkward way.
This is a picture that I attached in a thread about camping. I'm linking to it here. It will hopefully show up in the body of the post.
and this is a picture I took at the Baltimore Aquarium. I attached it to a post I made in a thread about close ups.
I have no idea how to do this without going back and finding images I had attached in previous threads. It would take some work. Maybe you could compose a post, upload an attachment and then delete the post before submitting it. Maybe the attachment would still be there and you could find it in the user cp.
Edit: I see that it didn't work. It can't be done. I used the [IMG] feature as well. Nevermind.