1. A slideshow generating apparatus comprising: a background moving-image storage device for storing background moving-image data representing a background moving image composed of a number of frames of a background image wherein an area in which a still image is rendered has been defined as a mask area; a still-image data input device for inputting still-image data; a scenario data storage device for storing scenario data that defines a position corresponding to the mask area in the background image, said position being one at which the still image represented by the still-image data that has been input from said still-image data input device is rendered; a positioning device for positioning the still image, which is represented by the still-image data that has been input from said still-image data input device, at the position represented by the scenario data stored in said scenario data storage device; a mask-image data storage device for storing mask-image data representing a mask image in which a still-image rendering area has been defined in association with the mask area of the background image, an area from which the still-image rendering area has been excluded being defined as a mask area; and a moving-image combining device for generating moving-image data for display by repeatedly executing, with regard to the number of frames of the background image, processing that combines the background image constituting the background moving image represented by the background moving-image data that has been stored in said background moving-image data storage device, the still image that has been positioned by said positioning device, and the mask image represented by the mask-image data that has been stored in said mask image-data storage device.