1. An image output system, comprising: an image capturing section which captures an image; a image capturing time measuring section which measures the time required for each image capturing operation of said image capturing section to capture the image; a ranking calculating section which ranks the image based on the image capturing time calculated by said image capturing time measuring section; an output controlling section which displays or prints out at higher priority the image ranked higher by said ranking calculating section; an auxiliary image capturing section in which a plurality of auxiliary images are captured sequentially at the timing before said image capturing section captures the image; and a composition change amount detecting section in which the amount of change in composition in the plurality of auxiliary images are captured by said auxiliary image capturing section; wherein, said image capturing time measuring section measures the time required for each image capturing operation of said image capturing section to capture the image based on the amount of change in composition detected by said composition change amount detecting section.