Visual Site Designer uses a file format called .vnu which allows you to create your own graphics and work in a WYSIWYG environment. You should never edit a .vnu file by hand. Every time you use the Preview or Upload tools, the program creates all the necessary HTML and image files for you.
Visual Site Designer automatically exports these files and images to the same folder every time you save. This folder has the same name as the .vnu file followed by _website (e.g. Mysite_website). If you wish to use an external FTP client to publish your site, make sure you upload all the files in this folder to your server.