ioanna, the thumbnail should come up by itself -- automatically -- when you use Upload image to post, like in your first example.
Do you mean that the Upload image to post feature automatically puts an [url] tag at the end, instead of the end tag [/url] ?!
If that is the case, then there must be a bug somewhere...
You can easily correct it by changing the last tag, from [url] to [/url] , before you click Submit.
But it is not supposed to be like that, of course.
If you mean something else, please explain again...
FoxLeonard