1. A method for generating a customized composite map image, said method to be implemented by a personal navigation device that has a map database established therein and comprising the steps of: a) receiving, by the personal navigation device, a user-specified input image of an area of interest, the user-specified input image being one of a landmark guide map, an establishment guide map, and a road map; b) obtaining, by the personal navigation device, from the map database a reference map that encompasses the area of interest; c) selecting, by the personal navigation device, a set of feature points in the user-specified input image and a set of map points in the reference map that correspond in geographical features to the feature points; d) automatically transforming, by the personal navigation device, the user-specified input image according to positional differences between said feature points and the corresponding map points so as to obtain a to-be-registered image having adjusted feature points corresponding in position to the map points; e) automatically overlaying, by the personal navigation device, the to-be-registered image onto the reference map in a manner that the adjusted feature points coincide with the corresponding map points to thereby obtain a customized composite map image; and f) outputting, by the personal navigation device, the customized composite map image; wherein, in step c), the feature points and the map points are automatically selected by the personal navigation device through pattern matching of the user-specified input image with the reference map according to a set of pre-defined user-configured selection rules; wherein the set of pre-defined user-configured selection rules defines at least one of a T-intersection, a Y-intersection, a main road, and a landmark, for pattern matching of the geographical features of the user-specified input image with the reference map.