var image = System.Drawing.Image.FromFile("...");
var bitmap = new System.Drawing.Bitmap(image);
var bitmapSource = Imaging.CreateBitmapSourceFromHBitmap(bitmap.GetHbitmap(),IntPtr.Zero,Int32Rect.Empty, BitmapSizeOptions.FromEmptyOptions());
bitmap.Dispose();
var brush = new ImageBrush(bitmapSource);