You can change your RegisterRoutes in below sequence then you will get your required output
routes.MapRoute(
"SinglePictureRoute",
"Pictures/{id}",
new { controller = "Home", action = "abcd", id = UrlParameter.Optional },
new { id = @"\d+" } // Parameter constraints
);
routes.MapRoute(
"UsersPicturesRoute",
"Pictures/{username}",
new { controller = "Home", action = "abcTest", username = UrlParameter.Optional }
);