//在指定位置并且按指定大小绘制原图片的指定部分
g.DrawImage(originalImage, new Rectangle(0, 0, towidth, toheight),
new Rectangle(0, 0, originalImage.Width, originalImage.Height),
GraphicsUnit.Pixel);
drawimage是缩放后绘图
如果你需要平铺或者截取,参考http://blog.csdn.net/htt9931/article/details/28644209