What jabjabjab is referring to is the custom background image zones can display in the alt-tab chat. It is indeed called chatbg.bm2. Since alt-tab chat is part of the frontend and not part of the game, it is necessary to include it outside of any lvz files, for example you would set your LevelFiles setting to chatbg.bm2 not somelvzcontainingchatbg.lvz. Also it must be a bmp file with the .bm2 extension.