small glitch in my new 2X1 doors and I am stumped

Hello, I am a new poster here.

I made my first few centered-on-two-tile doors the other day and they looked good and I felt great. It seemed too easy and I guess it was because a day later I discovered a small glitch. It only shows in game when the walls are down. It is like a little tab of drop down wall-top does not know it should not go there. I can't explain it well so please look at this pic:

Everything else is fine and it works and I am happy about that.

Curiously, the problem seems to happen when the wallmask PNG image is imported/exported to/from Paint.Net. I discovered this when I tried using the wallmask from Plasticbox's 2-tile Two-Panel Door. I imported the wallmask, as-is, into my "Plain Wooden Door 2-Tile" package and tested it in the game. Everything Looked good in the game --no glitchy thing. But the opening was a hair too wide so I opened up S4S and edited that same wallmask in Paint.Net. Well, it came back.

Any tips on what value I could edit to make it right, or what this unwelcome little tongue of wall is called? It is keeping my creation from being as nice as I want it to be.