Someday I really will write that help manual...
From the level list dialog (Level->Level List), pick a level and press the 'Properties' button. Select 'Wilderness', then load your background overland image using the button next to it. DC will automatically calculate the map width/height, as each map cell is 16x16 pixels.
Once loaded, the background image will appear on the map editor. You can then place zones, events, blockages, just as you would on a dungeon map.
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CocoaSpud
Dungeon Craft Development Team
http://uaf.sourceforge.net