Finally I have got it. An Avatar
Perhaps this may help some other desperate TMīs to make it work. At least you have sorted out one of possible bugs.
A lot of pics are stored in .jpeg format. In normal TM posts .jpeg works when surrounded with [img]. This "success" could make you wonder why your Avatar will not work (with .jpeg). Iīve tried it more than ones...
The TM Avatar ONLY (!!) accepts .jpg format for URL pics. Give it another try if this was the reason why.
At the end of the link Photobucket adds the following:
...after .jpg This must be removed and the line has to end with .jpg