1. A method for transmitting a plurality of pieces of image data by an image processing apparatus, the method comprising: receiving, by a transmission condition receiving portion of the image processing apparatus, a plurality of transmission conditions including a first transmission condition in which a first image format is designated and a second transmission condition in which a second image format is designated; obtaining, by an image data obtaining portion of the image processing apparatus, first image data; converting, by an image data conversion portion of the image processing apparatus, the first image data thus obtained into second image data corresponding to the first image format, and the first image data into third image data corresponding to the second image format; obtaining, by a private key obtaining portion of the image processing apparatus, a private key from either a portable recording medium or a recording medium provided in the image processing apparatus; saving the obtained private key to a storage portion provided in the image processing apparatus; generating, by an electronic signature generation portion, a first electronic signature corresponding to the second image data and a second electronic signature corresponding to the third image data by using the private key thus saved; and adding, after both the first and second electronic signatures for said second and third image data have been generated, the first and second electronic signatures to the second and third image data and transmitting the second and third image data.