Disclosed is an image data server which transmits image data through a communication network to a client apparatus for displaying an image on a display screen, the image data server including: an obtainment section to obtain aspect ratio information of the display screen from the client apparatus through the communication network; a specification section to specify a display screen area where the image based on the image data is not displayed in the display screen of the client apparatus when the image is output to the client apparatus according to the aspect ratio information obtained by the obtainment section; a creation section to create combined data by combining background image data corresponding to the display screen area specified by the specification section with the image data; and a transmission section to transmit the combined data created by the creation section through the communication network to the client apparatus.