Get yourself a Photobucket account.
Upload the pictures to photobucket
Copy the image URL of the picture you want to show (There are different ways to do this, but the easiest (And a way you can use on any picture on the internet) is to have the full size picture on display on your screen, hover the mouse arrow over it, right click and click Copy Image URL).
When replying to (Or starting) a thread; click on the little picture on the toolbar at the top of the post (Second row, 9th in from the right) and paste the image url in the pop up box. Click OK and when you post the picture should be in the post.