It is my understanding that you are not able to perform a multiplication operation (pixel*5) on individual pixels of your blocks. When you multiply your pixel by 5, it shows 255 every time.
This might be because the pixel values in MATLAB vary from 0 – 255 (0 being the minimum and 255 being the maximum).
When you try to modify a pixel to anything above 255, MATLAB will automatically keep the pixel at 255.
If your application requires you to scale a particular pixel by 5 times as compares to the other pixels,
you could scale all pixels down by a factor of 5 and then multiply the required pixel by 5.
I = imread('image_path');