1. An image sensing system comprising: an image sensing device for sensing the image of a subject and outputting image data representing the image of the subject; a preliminary-image-sensing control device for controlling said image sensing device so as to perform preliminary image sensing and output image data representing a preliminary image of the subject; a first target-image detecting device for detecting a target image from within the preliminary image of the subject represented by the image data that has been output from said image sensing device under the control of said preliminary-image-sensing control device; an exposure-amount adjustment determination device for determining whether adjustment of amount of exposure is necessary based upon the target image that has been detected by said first target-image detecting device; a first regular-image-sensing control device for controlling said image sensing device so as to perform regular image sensing and output image data, which represents a regular sensed image of the subject, in response to a determination by said exposure-amount adjustment determination device that adjustment of amount of exposure is unnecessary; an exposure-amount adjusting device for adjusting amount of exposure, in response to a determination by said exposure-amount adjustment determination device that adjustment of amount of exposure is necessary, in such a manner that the amount of exposure of the target image detected by said first target-image detecting device will result in a brightness in the vicinity of an appropriate brightness; a second regular-image-sensing control device for controlling said image sensing device so as to perform regular image sensing and output image data, which represents a regular sensed image of the subject, based upon the amount of exposure obtained by the adjustment performed by said exposure-amount adjusting device; a flash light-emission determination device for determining whether a flash light-emission is necessary based upon an image in an area corresponding to the target image detected by said first target-image detecting device and the amount of exposure obtained by the adjustment performed by said exposure-amount adjusting device in the preliminary sensed image of the subject represented by the image data that is output from said image sensing device under the control of said preliminary-image-sensing control device; a flash light-emission control device, responsive to a determination by said flash light-emission determination device that a flash light-emission is necessary, for controlling a flash light-emission unit in such a manner that a flash light-emission is performed in sync with regular image sensing by said second regular-image-sensing control device; a second target-image detecting device for detecting the target image from within the regular sensed image of the subject represented by the image data that is output from said image sensing device based upon control by said second regular-image-sensing control device; a tone-correction determination device for determining whether a tone correction is necessary based upon the target image that has been detected by said second target-image detecting device; and a tone correcting device, responsive to a determination by said tone-correction determination device that a tone correction is necessary, for subjecting the image data, which is output from said image sensing device under the control of said second regular-image-sensing control device, to a tone correction in such a manner that the target image detected by said second target-image detecting device will take on the appropriate brightness.