When a viewer resizes an image window while displaying image data distributed from a server on the image window of a client terminal, the client terminal notifies the server of the size of the image window after resizing. The server resizes the image data so that an image has the same size as that of the window, and transmits the resized image data to the client terminal. In this way, the need for the resizing processing in the client terminal can be obviated, thus reducing its load.