APNG loading and animating implementation for Imaging wasn't very hard work. However, there are not that many test images to be found on the Internet and most of the available ones are very simple. They're usually not using disposal methods and are basically just collection of independent images. Big difference compared to GIF files - some of them were quite difficult to animate right. I even got most of nontrivial APNG test files by converting some more complex animated GIF files. Tools for creating APNG images are not yet as sophisticated as some GIF animation tools - there's GIF to APNG converter, APNG Anime Maker, and some web based tools that assemble simple APNG file from bunch of uploaded single PNG frames.
Now I'm gonna extend PNG saver to allow saving multi images to simple APNG files, much like MNG saving works now. There's really not a good unified way how to pass some more information to image file savers in current library design - that's one of TODOs for new Imaging architecture.