Use the Image insert too. Find and click on the full size image. Click the Thumbnail Image tab. Set the size for the thumbnail. Click Cerate Thumbnail. Then check Auto-Link To Full Size Image. Then click Insert Thumbnail. The thumbnail will be inserted and will be linked to the full size image. The full size image will display in the center of the page when the thumbnail is clicked. For browser with JavaScript turned off, the full sized image will load in a new window when the thumbnail is clicked.
If you do not have ImageMagik
You can not create a new thumbnail image with the Insert Image tool. So you have to just insert an image - either a thumbnail you've created otherwise, or a resized display of the full sized image. You then have to use the Link tool to create a link from that image to the full sized image and make the link open intop a new window.
