You'll need some place to host your images from. Once you do - take the url (location or address of the image) and place it inbetween the
tags. I placed an extra space before the ] right bracket in order to show you what I'm referring to.
If you reply to a topic (instead of using the quick reply), there's a button you can click on which will write the img tags for you.
There's also information in the FAQ and if you click on the "Forum Code" link which is directly to the left of the message box (where you type in your message).
Hope that helps.