Just a heads up, since a lot of people might be interested in this:

Today only, Kenney's complete CC0 asset pack (normally available only to donors) can be downloaded from his site for free:

http://kenney.itch.io/kenney-donation

There's a free/open source pixel editor (aptly named "pixeditor") written in Python that could use a new maintainer.  You can see an old forum thread about it here.

It's stable and usable as it is, and has (in my opinion) about the most intuitive animation support that I've seen in any pixel editing program, but it's still in need of some more features.

Here's a relatively recent screenshot.  Notice the animation timeline.

If you're interested in contributing, the original author has offered to help someone get started.  You can find the git repository here.

Bart