Adding a small button IN the rss-files

I’ve been trying to find anything wbout how to do it right but I cant find anything, hope that anyone could help me out here.

I want to add a small 80×15 pixel button/banner in the rss-file, I think its possible, isnt it?

I understand that i have to edit 4 files,
wp-rds.php
wp-rss.php
wp-rss2.php and
wp-atom.php

Right?
But then I dont really know where to add the image code so it would work correctly. I want the small button to be shown under every entry.

Can anyone explain to me how to do it right?