Nicely done so far.
Perhaps you could also add the option to keep the backbround transparent. If it becomes black, you'll only be able to use the fire as a background. I'd love to put fire at the foreground as well as the background.
By the way, you might want to point out that this effect doesn't work with any older version of Paint.NET than 3.30. I tried it with 3.22, and it "crahed and burned" so to speak. :wink:
Cheers