There are two ways I can try to answer your question, actually. There's the far more complex question of how do you protect the content of your pages from prying eyes, and there's the simple question of how do you make it so people can't browse your "images" or "pics" directory.
Let's start with the first one. The essentially answer is that you cannot prevent people from manipulating your page, viewing the source HTML, JavaScript, CSS, etc., and even right-clicking on a graphic and using "Save Image As..." to grab a copy of what you have.
There are ways you can make it more difficult, including a common snippet of JavaScript code that disables the right-click feature while someone is on your page, but as with everything else that the Web site developer lives with, this is a case of "it's up to the browser". Specifically, if you can get something to appear on someone's screen, then they can take a screenshot, scrape the site with an archive program, or even browse their Web browser cache to get a copy of the image. For that matter, they can also ascertain the URL of your image and just reference that in an img src tag!
Here's a simple right click disable JavaScript snippet, by way of example:
So that, as I said, is the complex half of this question. The simple half is, well, simpler!
If you want to prevent people from browsing your "pics" folder, simply create an empty file called "index.html" and drop it in that folder. If someone tries to browse the folder, they jsut get a blank page, but you can manipulate its contents to your heart's content using sftp or ftp.
OR
instead of disabling right click
just give a hyperlink referece to some login page/authentication.html
Hope that helps you out!
whew! just recalled something....n i a\edited to answer
Another thing to do is to disable the directory listing in the web server software. Not everyone will have system administrator priviledge to do that, but if you do, it's a really simple setting.
That way, unless a visitor knows the exact file name or the directory has an index.html file, the server response will give a "file not found" error when they try looking.