The KS3 National Curriculum for Computing requires that you understand how data of various types (including text, sounds and pictures) can be represented and manipulated digitally, but it's also useful practical knowledge. If you didn't know how an image was stored, how could you make it, for example, lighter or darker? It would seem like magic.
On this page you can upload an image - it will read the values of red, green and blue values for the pixels from the file and display them below. You can then manipulate the image by adding to, or multiplying the values, either for a single colour or for all of them. What do you think will happen? Also, is the colour information all you need? NB. If you upload a large image there might be a delay before it is displayed.
You have not uploaded a valid file.