Naahh.. the link is there, it's just missing the opening "tag" (a piece of code that tells your web browser what to do with what's next).
When you click the "IMG" button, it should open a little box in which you can put the address of your picture. Then click OK
It should look something like this in your post
['IMG] then the address ['/IMG]
(mind I put in the little ' character to make you see the tags..) What's happening to your posts, somehow, is that you're missing the first one. No biggie...