Here is a quick guide.
This is where the image has been uploaded to: http://upload.lushstories.com/381-ANTH2.jpg
This is the URL I want people to go to when they click the banner: http://www.amazon.com/dp/B00838YM8C
First of all, highlight the image location, and then press the grey button with the red sun. That will make the forum software know it's an image.
Then highlight the whole line, including the [img and /img] codes, press the earth symbol with the chain on the bottom, and in the pop up window, put the destination URL, in this case, the Amazon one (you will probably be linking to your story on here).
That's it.
The resultant code should look like this:
