1. An image processing apparatus comprising: a first storage unit configured to store a plurality of images having the same contents and different resolutions as image files therein; an obtaining unit configured to obtain an area of a region to be processed in which a feature parameter is to be calculated and which is a portion of one of the plurality of images; a determination unit configured to determine a number of pixels which is required for obtaining the feature parameter in accordance with a type of feature parameter; a resolution calculation unit configured to calculate a resolution suitable for the area of the region to be processed obtained using the obtaining unit, in accordance with the number of required pixels; and a feature parameter calculation unit configured to select an image having the resolution calculated using the resolution calculation unit as an image to be processed from among the image files, and to obtain the feature parameter from a portion of the image to be processed corresponding to the region to be processed.