some improvements on tutorials

samples/cpp/tutorial_code/ImgTrans/Laplace_Demo.cpp

@@ -13,7 +13,7 @@ using namespace cv;
 /**
  * @function main
  */
-int main( int, char** argv )
+int main( int argc, char** argv )
 {
   //![variables]
   Mat src, src_gray, dst;
@@ -25,7 +25,12 @@ int main( int, char** argv )
   //![variables]
 
   //![load]
-  src = imread( argv[1], IMREAD_COLOR ); // Load an image
+  String imageName("../data/lena.jpg"); // by default
+  if (argc > 1)
+  {
+    imageName = argv[1];
+  }
+  src = imread( imageName, IMREAD_COLOR ); // Load an image
 
   if( src.empty() )
     { return -1; }