when you make a decal it brings up a small grey 2x2 2d box that you can drag and stretch to make the decal fit on it, and when you post the decal it would be on a 3d brick of the same size, not fitting perfectly on the screen. this would make it alot harder to steal decals by copying the image.
Proof im an ID God: http://web.roblox.com/Forum/ShowPost.aspx?PostID=136430669