An image transmission system includes: a client; a server; and a network connecting the client and the server. The client includes: an image request section that requests transmission of image data; an output instruction section that issues an output instruction for an image data file of a general purpose format; and an output section that outputs the image data file of the general purpose format specified as an output file. The server includes: a watermark insertion section that forms high-resolution data as an image data file to be transmitted and low-resolution data as a general purpose format file of image data to which an electronic watermark is given to specify the high-resolution data as watermark information whose resolution is reduced relative to the high-resolution data; and a distribution section that transmits the high-resolution data, the low-resolution data and a data selection program. The data selection program allows the client to detect existence of the low-resolution data to which the output instruction is made, and it specifies the high-resolution data from watermark information to designate the high-resolution data as the output file when the electronic watermark is detected and designates the low-resolution data as the output file when the electronic watermark is not detected.