l'solution

/* --std=c++11 */
#include <iostream>
#include <iomanip>
#include <cstdint>
#include <random>

//The solution
void rotate90clockwise(uint32_t* image, int size)
{
    for (int cycle = 0; cycle < size/2; cycle++)
    {
        int lastcol = size - cycle - 1;
        for (int col = cycle; col < lastcol; col++)
        {
            uint32_t tmp = image[cycle*size + col];
            image[cycle*size + col] = image[(size - col - 1)*size + cycle];
            image[(size - col - 1)*size + cycle] = image[lastcol*size + (size - col - 1)];
            image[lastcol*size + (size - col - 1)] = image[col*size + lastcol];
            image[col*size + lastcol] = tmp;
        }
    }
}

//To debug
void fillrand(uint32_t* image, int size)
{
    std::seed_seq seed = {1,2,323};
    seed.generate(image, image+size*size);
}

void printimage(uint32_t* image, int size)
{
    for (int i = 0; i < size; i++)
    {
        for (int j = 0; j < size; j++)
        {
            std::cout << std::setw(10) << std::hex << image[i*size+j];
        }
        std::cout << std::endl;
    }
}

int main()
{
    int N = 3;
    uint32_t* testimg = new uint32_t[N*N];
    fillrand(testimg, N);
    printimage(testimg, N);
    std::cout << "Rotate: \n";
    rotate90clockwise(testimg, N);
    printimage(testimg, N);
    delete[] testimg;
    return 0;
}