Well nick117, you're on the right track.
When you use the [ img] [ /img] tags, you have to post the URL of the page with the desired image.
Example: http://a0.twimg.com/profile_images/1239 ... bigger.png
= img url without [ img] [ /img] tags.
Then add the [ img] [ /img] tags without the spaces
at the start and end of the URL (the one with the forward slash always goes at the end).
And eureka; image.
= img URL with [ img] [ /img] tags.
If you want to post an attachment, you don't need the URL. You simply click the "Choose File" button from the bottom of the page after entering full editor mode. After this, it's pretty straightforward. Click the "Add the file" button and then the "place inline" button to have the line of code able to be placed where you want it. Or simply ignore the "place inline" button to let it appear at the end of your post automatically.
twitterLikwid_bigger.png [ 8.59 KiB | Viewed 1084 times ]
So basically, the img tags only work when you add a URL between them, and but attachments won't. When I used Awesome Face as an example, I had him on my desktop, so I was able to use him as an attachment. However; you didn't. Do when you posted the name with an image tag, it didn't work.
I hope this helps.