アプリ内のResourceをBitmapImageへ読み込む

var strmInfo = Application.GetResourceStream(new Uri("/Softbuild.OppaiIppai;component/Images/sample3.jpg", UriKind.Relative));
if (strmInfo == null)
{
    // エラー処理
    return;
}

var bitmap = new System.Windows.Media.Imaging.BitmapImage();
bitmap.SetSource(strmInfo.Stream);