@tinus53: Hello Bernhard,
To create a child theme you can use our plugin Generate Child Theme.
Install and Activate the plugin.
Then,
Go to => Dashboard => Generate Child Theme => Select Parent Theme as Catch Flames and fill in remaining fields and Click on Generate.
This will create and activate the child theme.
Now,
Go to => Child Theme Folder => Edit functions.php file and add the code in this link.
Now, the images in the feature content should appear in original size.
Let me know if this works out!
Kind Regards,
Skandha