How do you get a thumbnail for your thread?

I see threads all the time in “Finished Projects” that have a preview of the model they did as the thumbnail for there thread.

The thumbnail is of the image you have attached to your post. It will not work with linked images and only in specific finished art forums

How would i post an un-linked image?

Press the ‘Go Advanced’ button below the reply box to give added options including attaching images http://blenderartists.org/forum/faq.php?faq=vb3_reading_posting#faq_vb3_attachments