If you need to insert a thumbnail into your content, use the following url as the image source:
The ‘Size’ parameter can be set to Small or Medium, and the ‘ImageUrl’ parameter is the location of the image that you wish to generate the thumbnail from. This image must be located within the site collection. If the url to your full size image was http://servername/sitename/listname/filename.jpg, the value to pass here would be /sitename/listname/filename.jpg.
I’ve found this comes in handy when creating announcements with links to screenshots or photos, where you require an image to be dynamically resized to a thumbnail.