When you post a new thread or reply to a post using the reply button there is an "Attach file" section under the text box (below options). Click "Browse" and attach the image from your HDD. Jpeg or gif will post as a viewable image, other files such as texts and bitmaps will post as hyperlinks from which viewers can open or download the file.
To link to an image hosted elsewhere on the web, use the vBulletin [img] command or the IMG button when you post a reply using the reply button (as opposed to the quick reply). In between the open and close IMG commands should be a DIRECT LINK to the image itself (jpeg or gif).
Happy posting.
