Ok, this is the method that should work.
SetSubRect(const sfIntRect& SubRect)
sfImage image1;
sfImage image2;
image1.LoadFromFile("image1.bmp");
image2.LoadFromFile("image1.bmp");
sfSprite sprite(image1);
sfIntRect SubRect;
SubRect.Left = 0;
SubRect.Top = 0;
SubRect.Bottom = 50;
SubRect.Right = 50;
sprite.SetSubRect(SubRect);
sprite.SetImage(image2);