Upload the buttons via FTP to your images/misc directory. You may wish to create a new folder in there called navbar. Then use
Code:
<img src="$stylevar[imgdir_misc]/buttonname.ext" alt="whatever text is on the button" />
in place of the $vbphrase[whatever] for each link. If you put them in a folder called navbar then it will be
Code:
<img src="$stylevar[imgdir_misc]/navbar/buttonname.ext" alt="whatever text is on the button" />
You need to replace buttonname.ext with the filename of each button in turn.