Thumbnail screenshot of a webpage I am just wondering how to make a thumbnail of a webpage to display ? I've seen so many websites like stumbleupon , dzone have a thumbnail images of other websites. Is there any simple way to that ?

I believe BrowserShots or BrowsrCamp will do what you are after. Just visit those web sites, whack in the URL to the web page you wish to generate a thumbnail for, then let it do it's business.
Or, you could go with a more native approach by using the ScreenGrab Firefox plugin:

Screengrab! saves webpages as images…
It will capture what you can see in the window, the entire page, just a selection, a particular frame... basically it saves webpages as images - either to a file, or to the clipboard.

Alternativly, you could try FireShot as well

Easy Thumbnails

Easy Thumbnails is a popular free
  Windows utility for creating accurate
  thumbnail images and scaled-down/up
  copies from a wide range of popular
  picture formats.