One of the theme of 1984 is reality vs unreality. How can the symbol of a "mask" connect to this theme?
The image of a mask can connect to 1984 in a couple of ways. The first is that Big Brother seeks to place a type of "mask" over the people of Oceania. In seeking to convince the citizens that there is constant threat of war and that Big Brother and the Party know best, the mask of government control is being used to depict a state of "unreality." It is this "unreality" that the government wants to use to mask the condition of reality, one of oppression and intrusive control.
Another example of how the symbol or image of a mask can represent the theme of reality and unreality in Oceania would be Winston. As a character, Winston is driven to expose reality by tearing off the mask of Big Brother's control. His embrace of tactics of resistance is an example of how he wishes to tear off the mask. At the same time, his affair with Julia is done, in part, to challenge the establishment and remove the mask that Big Brother has placed over society. Winston wishes for reality to be exposed and to challenge the existing reality as "unreality." He is punished in the worst possible way for wanting to remove the mask that is imposed on Oceania.