构造一个跟picturebox1一样大小的Bitmap, 设置给picturebox1, 然后在上面画图
Bitmap image = new Bitmap(picturebox1.Size.Width, picturebox1.Size.Height);
Graphics device = Graphics.FromImage(image);
//如果picturebox1本身有内容,就先画到image上
device.DrawImage(picturebox2.Image, w, h); //用你想要的位置画picturebox2
picturebox1.Image = image;
picturebox有个属性可以调居中,平铺等等