re: From the Hubble site
The glare of the dwarf, which is 700 times brighter than the planet candidate (as seen at Hubble's near-infrared sensitivity) has been greatly reduced through image processing of NICMOS pictures taken at different Hubble orientations.
http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/03/image/a
So the brown dwarve isn't masked, its absence is a matter of the wonders of "image processing". I wonder what the images look like in their natural state...