1. An image-processing system operative to process image data obtained by capturing images outside a periphery of a vehicle, the image-processing system comprising: a plurality of image-capturing units that is affixed to the vehicle and that generates image-data items by capturing images outside the periphery of the vehicle; a bird's-eye-view-image-drawing unit configured to generate a bird's-eye-view image by determining a viewpoint above the vehicle for each of the image-data items generated by the image-capturing units based on the image-data item so that end portions of real spaces corresponding to two adjacent bird's-eye-view images overlap each other; an image-composition unit configured to generate a bird's-eye-view-composite image by combining the bird's-eye-view images with one another according to a predetermined layout; an object-detection unit configured to detect at least one object existing in at least one real space corresponding to at least one joining portion where the bird's-eye-view images included in the bird's-eye-view-composite image are joined to each other; a border-width-setting unit configured to determine a border image based on the overlap portions of the bird's-eye-view images included in the bird's-eye-view-composite image, the overlap portions being included in the joining portion, and to change a width of the border image when the object-detection unit detects at least one object existing in a real space corresponding to the overlap portion; and an image-addition unit configured to generate a border-and-bird's-eye-view-composite image in the joining portion by superimposing a border image having a width set by the border-width-setting unit on the overlap portion of one of the bird's-eye-view images; wherein the object-detection unit includes a sensor that is provided for each of the at least one real space corresponding to the at least one joining portion, the sensor operative to detect the at least one object existing in the real space, wherein the border-width-setting unit sets the width of the border image to a value smaller than an initial value when the object-detection unit detects the object existing in the real space corresponding to the joining portion, and wherein when generating the overlap portions of the bird's-eye-view images, the image-composition unit is configured to determine a bird's-eye-view image corresponding to the image-data item generated by the image-capturing unit having a more narrow view angle than the other image-capturing units generating the image-data items corresponding to the bird's-eye-view images to be the other bird's-eye-view image, and to draw the overlap portion of the other bird's-eye-view image with priority.