//I get image from simple resources (not an embedded)

//At first I've created converter from a Bitmap to BitmapImage
public static class BitmapSourceConverter
{
public static BitmapSource ConvertFromImage(Bitmap image)
{
return System.Windows.Interop.Imaging.CreateBitmapSourceFromHBitmap(
image.GetHbitmap(),
IntPtr.Zero,
System.Windows.Int32Rect.Empty,
BitmapSizeOptions.FromEmptyOptions());
}
}

//And usage
pushButton.LargeImage = BitmapSourceConverter.ConvertFromImage(Resources.buttonImage);
//where buttonImage is the image assembly resource which you can add via assembly properties.

//Here is highlighted code: http://pastebin.com/ZcQYN78v

//I think this code is more obvious and more simple than yours because there is no need to use assembly.

//Hope this information is useful.

//Best regards, Victor.