Requested feature overview description:
Small icons off to the side of a thumbnail to show that you previously upvoted/downvoted/favorited it.

Why would this be useful?
As far as I'm aware, there is no way to visually see if you voted or favorited a post. There are ways to see posts you voted on, using votedup/down, but those are limited to their own menus, and when searching tags, you can't see wether you did vote on something until you click on the post. A feature like this would allow you to easily identify posts you upvoted or favorited in the past to show you don't need to vote on them again, and you can easily ignore any posts you decided to downvote.

What part(s) of the site page(s) are affected?
Parts of the site where posts are displayed, and the post thumbnails.

Would definitely be nice, but may never happen.

If you're using a browser that's capable of installing user scripts, RE621 gives a thumbnail icon for whether you've favorited something or not, among many other things. But unfortunately it can't show the vote status due to to API constraints (topic #34507)

And for the more tech savvy, you can find data-is-favorited="true" or data-is-favorited="false" on the HTML tag if you were looking to create your own custom CSS or userscript.