1. An image monitoring apparatus, comprising: a singleton selector configured to select at least one singleton from among a plurality of objects in an acquired image; a single foreground image generator configured to generate a single foreground image of the singleton; an eigenvector extractor configured to extract an eigenvector of a foreground singleton contained in the single foreground image by using at least one selected from the group consisting of (i) a centroid of the foreground singleton, (ii) the number of pixels of the foreground singleton and (iii) coordinate values of the pixels; a vanishing point location calculator configured to generate a reference vector by connecting a predetermined point in the acquired image to the centroid of the foreground singleton, and determine the predetermined point as a vanishing point when the predetermined point minimizes an angle between the reference vector and the eigenvector; and a gradient determiner configured to determine a gradient of the foreground singleton using the vanishing point, and correct an inclination and a size of a bounding box of the foreground singleton.