This is a plugin for Paint.NET that automatically mirrors the right half of a player spritesheet onto the left half.
It has no concept of any horizontal anchoring of individual frames, so it assumes that all sprites are properly centered.
What does that mean?
Lay out your sheets properly!
If you do, this'll save you some busywork.
Or, to put it visually:
Credits to rocky for giving me the idea. He's doing the same thing in aesprite right now!