here is a new copy of the installed.png file to correct the problem, copy it to the following folder in your site:
admin_gallery/images/installed.png
also, edit the admin_gallery/global.php file, change lines 34 - 37:
require_once("images/installed.png");
//PowerGallery version
$wa_settings['pg_version'] = '1.2.0';
mysql_free_result($result);
to:
//PowerGallery version
$wa_settings['pg_version'] = '1.2.0';
mysql_free_result($result);
require_once("images/installed.png");