Moderators Note: This plugin may not work with newer versions of Paint.NET. An alternative plugin is available here:
https://forums.getpaint.net/topic/31629-animated-images/
Secure Download
If you get a warning, try the unencrypted download.
Here is a plugin to open and save animated GIFs/PNGs, one frame into each layer.
You will have to rename the files to .agif or .apng to be imported and rename them after saving.
[ I did this to