The image should be a multiple of 16 pixels in both directions. Each map square is 16x16. DC will compute map dimensions based on (image width)/16 and (image height)/16. Any extra pixels on the right or bottom won't be reachable by the party but they won't hurt anything. The maximum map size is 100x100 squares, and the largest overland map is 1600x1600 pixels.
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CocoaSpud
Dungeon Craft Development Team
http://uaf.sourceforge.net