How to add sub title name in the header using pixture reloaded sub theme of drupal 7.x?
The "sub title name" you are referring to is called the "Site Slogan" in Drupal.  In Drupal 7, you can add or edit your site slogan on the Site Information page (/admin/config/system/site-information).

Then, on the Pixture Reloaded settings page, just make sure that the "Site Slogan" checkbox under "Toggle Display" is checked.  You don't need any custom code.