The settings from your screenshot look correct. I’m fairly sure that if you copy/paste a PNG image into the editor, Discourse automatically converts it to a JPG. If that’s how you have been adding the image, try using the editor’s upload button to add the image and see if that makes a difference.
I don’t think the image preview jpg quality setting is relavant to the issue, but both the png to jpg quality and recompress original jpg quality settings need to be set to 100 to prevent PNGs from being converted to JPGs. Questions about conversion of PNG images have come up a few times. It would be good to find a way to make the settings that control this behaviour more obvious.