Editing feed-rss2.php

Hello, In order to show an image in my feed, i am modifying the feed-rss2.php file. I know this is bad practice and i should be using a filter, but i am not sure how.

I want this:

<content:encoded><![CDATA[]]></content:encoded>

to be:
[Code moderated as per the Forum Rules. Please use the pastebin]

any help would be appreciated.