TITLEPIC is an image lump used in all ZDoom games as the title screen. It can only be seen when the player is not currently in a level and hasn't started the game from a warp. However, if CONBACK is not specified the TITLEPIC lump will be used as the Console's background. TITLEPIC larger/smaller than the user's screen resolution will be scaled up or down accordingly.
The name of the lump can be changed with the TitlePage property in a GameInfo section. In Heretic and Hexen, the lump is named TITLE by default.