BitmapImage.Rotation Property

<Image Width="150" Margin="5" Grid.Column="0" Grid.Row="1">
  <Image.Source>
    <TransformedBitmap Source="/sampleImages/watermelon.jpg" >
      <TransformedBitmap.Transform>
        <RotateTransform Angle="90"/>
      </TransformedBitmap.Transform>
    </TransformedBitmap>
  </Image.Source>
</Image>

// Create Image element.
Image rotated90 = new Image();
rotated90.Width = 150;

// Create the TransformedBitmap to use as the Image source.
TransformedBitmap tb = new TransformedBitmap();

// Create the source to use as the tb source.
BitmapImage bi = new BitmapImage();
bi.BeginInit();
bi.UriSource = new Uri(@"sampleImages/watermelon.jpg", UriKind.RelativeOrAbsolute);
bi.EndInit();

// Properties must be set between BeginInit and EndInit calls.
tb.BeginInit();
tb.Source = bi;
// Set image rotation.
RotateTransform transform = new RotateTransform(90);
tb.Transform = transform;
tb.EndInit();
// Set the Image source.
rotated90.Source = tb;