Just now, we've added a section to most item pages in the Encyclopedia with some additional assets. Depending on the item, there are spritesheets, animated gifs and static images in .png format. We are making these available under a "Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported" license which means you can use them in your own projects.
You can read a bit more about this (and some other things, like the availability of end of the world notes) in this new blog post, A Post-Game Update from Tiny Speck. And, if you have questions or comments, this is the thread!
(p.s. We've also made an addition to your individual "archive" pages so that the complete set of spritesheets for each of your saved avatar customizations are available, instead of just the static png files)