Got your other message about posting.
If you want to start a new post, hit the 'new topic' link at the top of the topic list page. When you are done with writing you message, scroll down and hit the 'manage attachments' button. In the screen that comes up, hit 'browse' (you have to save a picture to your computed before you can download it) then download.
To attach a pic to a response to a thread, hit the 'post response' button while in the thread and follow the same proceedure.