http://www.flashbang.se/archives/155

screenshot.cpp

void screenshot(char filename[160], int x, int y)
{
    // get the image data
    long imageSize = x * y * 3;
    unsigned char *data = new unsigned char[imageSize];
    glReadPixels(0,0, x,y, GL_BGR, GL_UNSIGNED_BYTE, data);

    // split x and y sizes into bytes
    int xa= x % 256;
    int xb= (x - xa) / 256;

    int ya= y % 256;
    int yb= (y - ya) / 256;

    // assemble the header
    unsigned char header[18] = {0,0,2,0,0,0,0,0,0,0,0,0,(char)xa,(char)xb,(char)ya,(char)yb,24,0};

    // write header and data to file
    fstream File(filename, ios::out | ios::binary);
    File.write(reinterpret_cast(header), sizeof(char)*18);
    File.write(reinterpret_cast(data), sizeof(char)*imageSize);
    File.close();

    delete[] data;
}