If you want to add a picture from your computer, then first you need to upload it somewhere,
example: DeviantArt, Tinypic, Photobucket
But if you already have your picture uploaded, you right click on the it and select 'copy image URL'
When you have done that, make a thread, copy the link there.. and add img in front of the URL and /img behind it. The img and /img, have to be inside the: [ and ]