Just wanted to echo that this is really awesome. Thanks!
I was trying to get the explosions generated from this program to be transparent, and your plugin was exactly what I needed. I dropped it in the C:\Program Files\Paint.NET\Effects folder, loaded up the program, loaded an explosion image, copied it to the clipboard, ran your plugin and BLAMMO! Instant alpha. Beautiful.
Thanks again, this saved me a lot of time and headache.