the file uploaded onto wordpress and selected as thumbnail has a transparent background, if it does not by the time it is displayed as a related areticle thumbnail it is the theme or WP changing that, see image file
http://bitcoin-betting-guide.com/wp-content/uploads/2014/05/tagger-NFC-wallet-frame.png
http://bitcoin-betting-guide.com/?attachment_id=6498
Almost all other thumbnails that display the same way in the related articles section you can see have transparent backgrounds by click though to that article as the thumbnails are the image that is displayed on that page (not the case with the example you got but is the case with the others)