Well, since you already understand the use of IMG button, so it will be easier to explain to you....
goto a free hosting site, like
www.tinypic.comUse it to upload the pictures you want, click on Browse, locate the file and then click Host, it will host the file online and will give you its URL, you can then copy+paste the URL's to your topic, (there are many URL's given, one of which is already tagged with [I M G].... [/ I M G] so it would be easier for you to just copy that without using the IMG button.
Hope that was clear.
Take Care