ABOUT
A Dragable or Transparent image is a type of picture often found on Tumblr dashboards. They are often .png images but some times animated .gifs. Both of these image types have a special property which allows them to have transparent backgrounds/areas within them. This property is exploited by Tumblr users where they hide messages or images (masking them in dashboard white) within an uploaded image so it looks innocent or blank. Its only when the image is dragged onto a darker background that the user finds out that there is a hidden message.
More inventive users make animated gifs which instruct you to grab the image and drag it to determine an out come or spoilers i.e. your death, your future lover etc.
http://www.tumblr.com/tagged/transparent