Yeah your right about the jaggies, it looks like when someone turns on Edge Enhancer on their HDTV and it starts showing compression jaggies.
I just haven't quite gotten the hang of GIMP yet. It's powerful, but also not so simple to use(for a lay). So many choices and many I don't know what they do, and the ones that I do know have waaay to many choices.
I've learned a decent amount of the Gimp options I need, but I still don't know many of other options either.
Start with the basics you need: scales, transforms and in time you'll be up using more functions.
Horace have you already taken a look at this thread, it has to do with custom tile problems: Irritating little icon glitch - Windows 10 Forums
Just did one for Defender now too. one question, which is the name that shows on the tile, special when the .exe file name does not match the program name. like in this case instead of Defender the .exe file is called MSASCui so I made EVERYTHING it the tile pack use that. Which is the one I can rename, to show the actual program name, on the tile.