I don't know how we'll cover all tracking images, but for some common ones like Medium's, it'd be good to block them. Medium includes an image with a URL like:
https://medium.com/_/stat?event=post.clientViewed&referrerSource=full_rss&postId=<ID>
We could either never add them to the page, or we could make sure that the image proxy masks the user's IP / User-Agent and doesn't send cookies. I think this may already be the case.