To create a thumbnail version of an image you have placed on a web page:

   1. Open the Visual Style Editor from the pages section.
   2. Click on the 'Insert Picture' icon from the Toolbar.
   3. Choose the image and the desired thumbnail size (from small to large or custom sized).   
   4. Click Yes.

To create a thumbnail from the context menu:

   1. Go into the Visual Style Editor.
   2. Right click at the desired destination on your page.
   3. From the Context Menu choose Insert -> Thumbnail  

Toolbar Icon: