I inserted image thumbnail. But when I preview it is not displayed as thumbnail.
When you insert an image in a post, please select the Large size setting. If you click on your image you will see that it is undreadable.
Did you select the Show Thumbnail check box in the preset?
Also it only shows as a thumbnail when you generate. It shows full size in Preview as the thumbnail code is added when the preset runs.
My site www.grainge.org includes many free Authoring and RoboHelp resources that may be of help.