User Interface (UI) image that is used in terrains selection screen is 512×128 resolution, it has alpha channel which can be used. BIS chernarus image name is ui_selectisland_chernarus_ca.paa and is called from the config.cpp as follows:
icon = ""; pictureMap = ""; pictureShot = "\ca\chernarus\data\ui_selectisland_chernarus_ca.paa";