A server apparatus renders a process result of software on an image memory that stores a display image to be displayed on a client terminal, detects an update area in which an update occurs between frames of an image. The server apparatus selectively switches and performs a first generation process and a second generation process according to the number of detected update areas. The first generation process is a process that generates a color map for each update area based on colors used in the update area. The second generation process is a process that generates a color map commonly used between the update areas based on colors used in images of the update areas. The server apparatus reduces the number of colors in an update area using the generated color map and transmits a compressed image of the update area to the client terminal.