1. An image sensor chip comprising: a first wafer including an image sensor having a plurality of sub-pixels, each of the plurality of sub-pixels being configured to detect at least one photon and to output a sub-pixel signal according to a result of the detection, the plurality of sub-pixels being grouped into first through n-th sub-pixel groups, each of the first through n-th sub-pixel groups including at least one of the plurality of sub-pixels; and a second wafer including an image processor, the first wafer and the second wafer being formed in a wafer stack structure; the image processor being configured to process sub-pixel signals for each of the plurality of sub-pixels to generate first through n-th sub-pixel images, the first through n-th sub-pixel images corresponding to the respective first through n-th sub-pixel groups; perform scaling and registration on the first through n-th sub-pixel images to generate a high-resolution image; scale the high-resolution image to a size of at least one of the first through n-th sub-pixel images; compare the scaled high-resolution image with one of the first through n-th sub-pixel images; and correct a reconstruction error based on the comparing.