how to reflect the changes in the image

am implementing a small algorithm in Gray scale image.. since the array size of an image is too long to implement i am retrieving some small array from the large array of the image. then after implementing the logic of the algorithm with the pixels of the image in small array i need to place it back to the large array such that when i read the image again i need to get the changes of that pixels i did... i found that i did not get the changes in the image array.. but my logic is correct with small array i worked with. how can i modify the pixel changes in the image array