Just look in your theme.php file for something similar to this
Code:
."<td>\n"
."<a href=\"index.php\"><img src=\"themes/Reverie/images/logo.gif\" align=\"left\" alt=\""._WELCOMETO." $sitename\" border=\"0\"></a></td>\n"
logo.gif, that is your image.
Now you just need to add <center> tags to either end, like this.
Code:
."<center><td>\n"
."<a href=\"index.php\"><img src=\"themes/Reverie/images/logo.gif\" align=\"left\" alt=\""._WELCOMETO." $sitename\" border=\"0\"></a></td></center>\n"
Make sure when you make any changes to ANY files, that you back up the original before you replace it on your serer with your modified one.
mikem