But the “solution” you linked adds borders to every image, which most sites wouldn’t want. Your problem isn’t with Discourse, it’s with the way in which images are resampled when ImageMagick scales them down. Adding a border to the image has more of an impact than the CSS change, which by your own admission you don’t want.
Why not approach ImageMagick and see if they have a solution rather than blaming Discourse, which isn’t responsible. This isn’t a Discourse problem in the slightest.