1. An image supply device used with an image display device adapted to display an image on a display surface and an operation detection device adapted to detect an indication location in accordance with a position indication operation on the display surface, comprising: a drawing section adapted to generate a superimposition image to be displayed while being superimposed on an original image, the superimposition image being generated along a trajectory of the indication location detected by the operation detection device, the generation along the trajectory occurring using a trace line and based upon a response to a user action; a first image generation section adapted to generate a first composite image by superimposing the superimposition image generated by the drawing section on a first original image; a second image generation section adapted to generate a second composite image by superimposing an operating image and the superimposition image on a second original image, the second composite image being generated in response to a change of the original image from the first original image to the second original image, the operating image being used for instructing erasing the superimposition image, the second image generation section displaying the operating image when a difference between the first original image and the second original image exceeds a threshold level; and an output section adapted to output the second composite image to the image display device, wherein the image supply device reproduces a file including a plurality of slides, and the first original image and the second original image correspond to slides of the plurality of slides included in the file.