1. An object detection system, comprising: a terminal device and a server, the terminal device including an imaging device configured to capture images, a first image processing device configured to receive the images from the imaging device and detect from the images an image that contains a target to be detected, and a first communication device configured to transmit data to and receive data from the server, and the server including a second communication device configured to transmit data to and receive data from the terminal device, a second image processing device configured to recognize the target to be detected from the image received from the terminal device, and a storage device configured to store data on the recognized target to be detected, wherein the terminal device is configured to execute: a process of, with the first image processing device, detecting from the captured images a candidate image that contains the target to be detected on the basis of a first evaluation criterion, and a process of, with the first communication device, transmitting to the server the candidate image detected on the basis of the first evaluation criterion, and wherein the server is configured to execute, a process of, with the second communication device, receiving the candidate image, a process of, with the second image processing device, recognizing from the candidate image the target to be detected contained in the candidate image on the basis of a second evaluation criterion that is different from the first evaluation criterion, and a process of storing an image of the target to be detected and the candidate image into the storage device, wherein the first image processing device is further configured to execute: a process of converting a color space of the image containing the target to be detected, and acquiring color information on the converted color space, a process of calculating, for the image containing the target to be detected, a first average value indicating an average value of lightness of the color information in a region of the target to be detected, a process of comparing, for the region, lightness of the color information on each pixel with the first average value, and generating a corrected image with corrected lightness/darkness, a process of extracting the region of the target to be detected on the basis of the corrected image, and a process of detecting the target to be detected in the region, a process of calculating, for images captured in the past prior to the image containing the target to be detected, a second average value indicating an average value of lightness of color information in a region corresponding to the region of the image containing the target to be detected, and generate the corrected image based on a proportion of the first average value and the second average value, a process of detecting the image containing the target to be detected by estimating an ellipse in the extracted region of the target to be detected, wherein the server is configured to execute: a process of identifying the target to be detected by comparing the target to be detected with a reference image prepared in advance, a process of generating alert information corresponding to the recognized target to be detected, and transmit to the terminal device the alert information and an image of the recognized target to be detected, and wherein the terminal device is further configured to display on a display screen the image of the target to be detected received from the server and output the alert information.