You can show the page feature image overridden by the BuddyBoss Theme via the child theme. Follow the below process to override,

• Kindly copy the page.php from the buddyboss-theme inside your child theme
• After that, you need to add your thumbnail code into page.php below I have added the code
  
  

if (has_post_thumbnail()) {
   the_post_thumbnail('large', array('class' => 'your-custom-class'));
}

page.php look like the below screenshot