Form1Load(object sender, EventArgs e)
{
     Random rnd_ = new Random();
     ShowRandomImage();
}

ShowRandomImage()
{
     ShowImage(rnd_.Next(maxImageIndex));
}

ShowImage(int index)
{
.............
}

bool TestAnswer(....)
{
     bool otvetIsPravilniy = false;
     ..........
     if (otvetIsPravilniy)
      {
           ShowRandomImage();
       }
}