It's pretty simple.

First, you can't use FAST REPLY you either have to create a new thread, or lick the REPLY icon on the upper left corner of the post you want to reply to. (you can also click EDIT on an existing post of yours)

Below and to the left of the blank post area you will see a link to add attachments, navigate via windows to the picture you want using the BROWSE button, and click OK when you've selected up to 8 or 10 images (i forget what the limit is per post but i think it's 10)

If the file isn't too big you'll get a message that says FILE UPLOADED SUCESSFULLY