WriteableBitmap wrBitmap;
BitmapData bdata = new BitmapData();

void Test(Stream stream)
{
    var gdipBitmap = new Bitmap(stream);
    var width = gdipBitmap.Width;
    var height = gdipBitmap.Height;
    if (wrBitmap == null || height != wrBitmap.PixelHeight || width != wrBitmap.PixelWidth)
    {
        wrBitmap = new WriteableBitmap(width, height, 96, 96, PixelFormats.Pbgra32, null);
        display.Source = wrBitmap;
    }
    wrBitmap.Lock();
    bdata.Width = width;
    bdata.Height = height;
    bdata.PixelFormat = System.Drawing.Imaging.PixelFormat.Format32bppArgb;
    bdata.Scan0 = wrBitmap.BackBuffer;
    bdata.Stride = wrBitmap.BackBufferStride;
    gdipBitmap.UnlockBits(gdipBitmap.LockBits(
        new System.Drawing.Rectangle(0, 0, bdata.Width, bdata.Height),
        ImageLockMode.ReadOnly | ImageLockMode.UserInputBuffer,
        System.Drawing.Imaging.PixelFormat.Format32bppArgb,
        bdata));
    wrBitmap.AddDirtyRect(new Int32Rect(0, 0, width, height));
    gdipBitmap.Dispose();
    wrBitmap.Unlock();
}