No, I am and was not offended, just had very little time. Here is my approach. I demonstrate by using an image from the getpaint.net website:
1. Open Image and select an area with S (Shortkey)
2. Highlight the area to be removed
3. Remove area with DEL / ENTF (in Germany)
4. This is how it looks in the end
5. Save image...
6. ...as PNG-file
7. And now open the picture with Windows Photos and the white borders are visible.
8. Zoomed in...
As already written, these edges are also visible in the game. Interestingly, they are not visible with IrfanView or XnView. Only with Windows Photos and in the game (Quake III Arena).
Thank you for the help and look forward to your answer. You can test yourself with any picture at any time. With the GIMP, these borders do not appear. "Antialiased / Pixelated selection quality" does not change anything.
Download: Output files from Paint.Net and GIMP
Best regards
ZerTerO