1. A method of locating fingernails of a person's hand in an image, comprising the steps of: a) storing respective individual representations of at least a plurality of each digit of a person's hand, each individual representation including data defining: a respective digit-shape model and a corresponding function that approximates variations to the digit-shape model; and a fingernail sub-shape model comprising a subset of the data defining the respective digit-shape model, and a corresponding function that approximates variations to the fingernail sub-shape model; b) storing a global representation of the plurality of digits of the person's hand, including data defining a global model defining a non-overlapping arrangement of said digit-shape models, and a corresponding function that approximates variations to the global model; c) determining an approximate position of each identified fingernail in the image; d) determining an approximate position of each digit in the image, based on the global representation and the approximate position of each identified fingernail; e) refining the approximate position of each digit in the image, based on the respective digit-shape model and its corresponding function; and f) refining the position of each fingernail in the image, based on the respective fingernail sub-shape model and its corresponding function.