Hi, if I understand you correctly, you are trying to write not only the post title, but also more text (such as the name of the website) to the featured image. Correct?
Unfortunately, this isn’t already an option in the free version, but it is an option if you were to upgrade to the PRO version, which would allow you to add additional characters both before and after the text you chose to write to the image. You might consider checking out the free trial to see if it does what you want.
To code it into the free version, you would need to add something like this around line 201.
$auto_image_post_text = 'Site Name : ' . $auto_image_post_text;
Hope that helps!