1. A non-transitory computer-readable storage medium having stored therein an overhead map resource generation program which is executed by a computer of a map data resource generation apparatus for generating an image data resource to be used in displaying a two-dimensional map corresponding to an overhead view of a particular region of a virtual three-dimensional space, the overhead map resource generation program causing the computer to perform operations comprising: acquiring overhead images of a plurality of sub-regions of a predetermined region in the virtual three-dimensional space by shooting separate images of each sub-region from a viewpoint of an overhead virtual camera provided in the virtual three-dimensional space; determining whether color information included in the acquired overhead image satisfies a predetermined condition; and generating map image resource data by storing and accumulating, in a predetermined storage medium, only acquired overhead images which contain color information that does not satisfy the predetermined condition, wherein information processing operations performed for said acquiring, determining and generating map image resource data are repeated for each of said plurality of sub-regions until overhead images of all sub-regions comprising the predetermined region in the virtual three-dimensional space are processed.