1. A method for transmitting a plurality of pieces of image data, the method comprising: causing an image processing apparatus to perform: a process of obtaining a private key, a process of generating individual electronic signatures for the plurality of pieces of image data by using the private key thus obtained, and a process of adding the individual electronic signatures to the plurality of pieces of image data and transmitting the plurality of pieces of image data, the process of adding being started after the individual electronic signatures for said plurality of pieces of image data have been generated.