Ok I just modified the code.
Please make these changes and you should be fine.
File: /Users/giri/Sites/wordpress/wp-content/themes/supreme-directory/inc/geodirectory-compatibility.php
In line 821 you will see this line.
https://github.com/mistergiri/supreme-directory/blob/master/inc/geodirectory-compatibility.php#L821
$entry_author = '<img src="'.get_stylesheet_directory_uri() . "/images/gravatar2.png".'" height="100" width="100">';
Replace that line with these two lines.
if (!defined('SD_DEFAULT_GRAVATAR_IMAGE')) define('SD_DEFAULT_GRAVATAR_IMAGE', get_stylesheet_directory_uri() . "/images/gravatar2.png");
$entry_author = '<img src="'.SD_DEFAULT_GRAVATAR_IMAGE.'" height="100" width="100">';
Now all you have to do is that add this line in your wp-config.php file
define('SD_DEFAULT_GRAVATAR_IMAGE', 'http://your image url here');
Thanks