Thumbnail can be generated from a bitmap (.BMP) or a JPEG (*.JPG) or even a EMF (Encapsulated Meta File).
When thumbnails are created and embedded into your PDF files, you will see the thumbnails displayed on the left column of your PDF Reader or Viewer (usually Adobe Reader).
With this freeware utility, you can decide whether to show thumbnails when PDF file is opened.
Behind Asterisks XP
This nifty software allows users to decrypt passwords hidden under asterisks
"*" characters. It works on any platform and gets passwords even on web pages. More info...