This option should contain the local file system path to the web servers wwwroot. It will automatically detect the wwwroot by default but some very specific server setups might need this one to be defined. The wwwroot should be the point http://www.site.com/ points to in windows this is normally "c:Inetpubwwwroot" or in Linux it might be "/var/www" this depends on the server setup.
This setting is used to map file paths such as "c:/Inetpub/wwwroot/somedir/somefile.gif" into URLs such as "http://www.mysite.com/somedir/somefile.gif" it's doing this by taking the path and removing the wwwroot part.
So for example if the preview.wwwroot is configured to "c:/Inetpub/wwwroot" it will then take a path like this "c:/Inetpub/wwwroot/somedir/somefile.gif" and convert it by removing the first part and the result would be "/somedir/somefile.gif" it will then use other options such as preview.urlprefix and preview.urlsuffix to build a complete URL such as "http://www.mysite.com/mydir/myfile.gif?somesuffix=x"
If you are using virtual directories, you must configure this option to the point where the files for the virtual directory is stored. For example "d:/myroot/mydir" if the virtual directory points to "d:/myroot/mydir".
Example of usage in the config.php file
$mcImageManagerConfig['preview.wwwroot'] = "/var/www";
Example of usage in the Web.Config file
<add key="preview.wwwroot" value="c:\Inetpub\wwwroot" />
User Contributed Notes
If hosting from an user directory (eg http://localhost/~johndoe/foo/ on *nix) then this needs has be set as the wwwroot is incorrect path.
$mcImageManagerConfig['preview.wwwroot'] = '/home/johndoe/public_html/foo/';