一、本地图片文件

ImageBrush imageBrush = new ImageBrush();
imageBrush.ImageSource = new BitmapImage(new Uri("bg.jpg", UriKind.Relative));
button.Background = imageBrush;

二、内存中的图片

//此处图片从文件中读入用以模拟内存中的图片

System.Drawing.Bitmap bitmap = new System.Drawing.Bitmap("bg.jpg");
MemoryStream stream = new MemoryStream();
bitmap.Save(stream, System.Drawing.Imaging.ImageFormat.Png);
ImageBrush imageBrush = new ImageBrush();
ImageSourceConverter imageSourceConverter = new ImageSourceConverter();
imageBrush.ImageSource = (ImageSource)imageSourceConverter.ConvertFrom(stream);
button.Background = imageBrush;