JPG to RAR

I'm trying to put a RAR file into a jpg and am having a problem. I do the whole "copy /b picture.jpg + archive.rar thing.jpg" thing, but for some reason whenever I try to open in WinRAR it says "Could not open because it is in an unknown format or damaged."

What the fuck am I doing wrong?

I'm Talking about this:

http://www.thetechtray.net/blog/2007/12/10/hiding-files-in-jpeg-images/

I follow all the directions on that but when I try to open the file in winRAR it gives me an error message