1. An image processing system, comprising: a superimposing merger operation unit to receive encoded data of a main image and encoded data of at least one sub image and generate superimposedly merged encoded data by merging the encoded data of the main and sub images losslessly via superimposition, wherein the merged encoded data generated by the superimposing merger operation unit is configured so that the main image is stored therein and that when the merged encoded data is decoded, each of the main image and sub image is reproduced with the sub image being reproduced so that the sub image is superimposed with respect to the main image; and an encoded data generation unit to generate, from encoded data of an image whose code string is divided into units of areas, encoded data including a code string of a selected one or more of the units of the areas, wherein the encoded data generated by the encoded data generation unit is input to the superimposing merger operation unit as the encoded data of the sub image.