This forum uses vB code however so …
[.url=http://www.yourpage.com/yourimage.jpg][.img]http://www.yourpage.com/yourthumb.jpg[./img][./url]
(Just remove the red dots)
The other option is to attach files using the [Manage Attachments] button listed under “Additional Options” when making a new post. Upload your full size image and a thumbnail is auto created.