Switch quantity not an integer

What should I do to fix this error: "Switch quantity not an integer" with this piece of code?

Code:

if(argc == 2){
        switch(argv[letter]){
            case 'g':
                for(j = 0; j < (width * height); j++){
                        scanf("%c%c%c", &image[j].r, &image[j].g, &image[j].b);
                }
                i = greyscale(image);
                break;
            case 'r':
                for(j = 0; j < (width * height) - 1; j++){
                        scanf("%c%c%c", &image[j].r, &image[j].g, &image[j].b);
                }
                i = rotation(image);
                break;
            default:
                printf("%c%c%c", image[j].r, image[j].g, image[j].b);
            }
        }

It's for a programming assignment and I'm supposed to execute the program like this: ./a.out r < ****.ppm(for rotation) or ./a.out g < ****.ppm (for greyscale).

O_o

If you have a `char **' and dereference it you get a `char *'.