JPG Background image without dither

[FokkoVeegens]

I created a new background image that works for badge.py, based on the provided back.jpg. The original back.jpg is perfect without any dithering (jpg compression artifacts). Whenever I try to save my own jpg, I never get it as perfect as the original image. I converted the image to a BMP and made sure I removed any pixels that should not be there. Then I used magick to convert it back to jpg using the following command:

 magick -density 1200 -monochrome +dither +compress -quality 100 -sampling-factor 4:4:4 back.bmp back.jpg

But even then, the compression artifacts re-appeared;

The result is something like this:

How can I fix this, or in other words: how did you guys make the back.jpg without any dither/compression artifacts?