资源介绍
在图片中添加文字，并合成新的图片。
BitmapSource bgImage = (BitmapSource)img.Source;
RenderTargetBitmap composeImage = new RenderTargetBitmap(bgImage.PixelWidth, bgImage.PixelHeight, bgImage.DpiX, bgImage.DpiY, PixelFormats.Default);
double scaleX = bgImage.Width/imgbk.ActualWidth;
double scaleY = bgImage.Height/imgbk.ActualHeight ;