Thank you Kamar. But you did not get it right. I have already georeferenced the picture which was in png format. Let say this picture displays the annual precipitation. Each color code must represent a certain value or range of precipitation, but this is not the case since it is just an image. I want now to change (using GIS operation) "inform" the GIS package that "yellow color = 1000mm to 1200mm precipitation, red color=800mm to 999mm, blue color=600mm to 799mm, and so on. I know that each color has a Digital number on a png or jpg image. We can also make these change using the Digital number. But how to do it? Attached is one of the pictures I need to reclassify.