Referrer will be passed. You can choose to not pass referrer to a remote server.
Quickly test if the image can be hot linked.
Quickly test if the image has any transparency.
Quickly test how a transparent image or icon looks on a color background.
Disable referrer: http://savanttools.com/testimg.asp?referrer=no
Link with image: http://savanttools.com/testimg.asp?img=some_image_url