marty-harty I updated the solution here:
You will need to update 2 files:
/templates/ja_stark/html/layouts/joomla/content/masthead.php
/templates/ja_stark/html/com_content/article/project.php
Note that if you want to set fulltext image as background image of masthead section, you open the file: /templates/ja_stark/html/com_content/article/project.php
Look for this line of code:
$imagesBg = $images->image_intro;
change it to:
$imagesBg = $images->image_fulltext;