Image Processing is algorithms performed on images to either enhance or isolate information from image. Digital image processing uses one or many from vast range of algorithms, depend on application, on digital image or set of images.
Shader is as its name suggests, used for shading-an adjustment of brightness and color, it is typically a computer application/program, which now-a-days also used to provide various special effects to image/video.
GPU is Graphics Processing Unit, which generally used in computer systems to accelerate graphics related tasks which are compute intensive. GPU has parallel processing units which increase computation power to many folds and very useful in image processing intensive tasks. Typical example is computer/mobile phone game, which requires very heavy image processing related computation and GPU helps running graphics intensive application lag free.