1. A document server for transmitting a document consisting of text only, wherein the document server transmits, before a terminal sends a request for an image to an image server, the document containing or specifying a program for causing the terminal to function as: an additional information extractor that extracts, as additional information, characters or character strings that are added to a background image from the document or the program specified by that document in which characters or character strings are written at a position of a target to be transformed by transformer; an image location information generator that generates image location information specifying the image server as a request destination, and containing an image parameter that designates the extracted additional information and the background image; and an over-writer that overwrites an image location information contained in the document with the image location information generated by image location information generator.