1. An information processing device comprising: an image display unit configured to display one or more main images within a main area in a screen and one or more sub images within a sub area in the screen, each of the sub images being associated with one of the main image; a first display control section configured to display other main image within the main area in the screen according to a user's instruction so as to replace the main image that is already displayed with the other main image, and display other sub image within the sub area in the screen in addition to the sub image that is already displayed, the other sub image being associated with the other main image; a designation receiving unit configured to receive a designation about any one of the sub images that is displayed in the sub area; and a second display control section configured to remove one or more sub images that are displayed after the designated sub image is displayed, and display the main image associated with the designated sub image again in the main area instead of the main image that is lastly displayed.