1. An image-processing system processing 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; a posture-change-detection unit configured to detect an amount of change in a posture of the vehicle; a border-width-setting unit configured to determine a border image based on the overlap portions included in a portion where the bird's-eye-view images included in the bird's-eye-view-composite image are joined to each other and to change a width of the border image based on the detected vehicle-posture-change amount; and an image-addition unit configured to draw 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.