First thing is that your image must be hosted on a website somewhere, like photobucket or anywhere else. As far as I know "attached" pictures are being uploaded right to Talkbass.com and stored on their server while the other kinds of pics that you're asking about are already stored on a "remote" server aka. another website..
To make a pic appear in your thread the first thing that you need to have is a unique URL to where the picture is stored.
For example I just Google'd "bass player" and found an image to use. I copied the link to where that image is hosted, the link below is the one I'm talking about
:
http://www.merryswankster.com/images...s%20player.jpg
Now, if you take that same link and wrap it with the [*IMG*]insert link here [*/IMG*] tags (remove the asterisks) you will get this:
You can also click the little yellow button that looks like it has some mountains on it with a little round circle floating above them. That's the button to "insert an image".. Just click the button and it will ask you for a URL, insert your link and voila, you have an image!
I hope that helps and makes sense!