Adding Custom Fields to Image Widget

Hi guys!

I have been searching around all the documents in this forum and really struggling to get this working. Every time I try something from the documentation my site crashes.

All I am trying to do is add a ‘subtitle’ field in the image widget. Please can someone help with what I need to do to make this work?
What code is it I need to put in my functions.php file to make this extra field appear?

Thanks in advance! :)