C# runs on the server, JavaScript in the browser. When you want to make it impossible to right-click an object (e.g., to make it 'impossible' to save a picture), you HAVE TO use JavaScript.
But even then, it's pretty easy to use a browser that will allow right-clicking all the same, OR to simply look in the browser's cache for the picture one wants. So you might as well surrender to the fact that people will copy your stuff.