I am not sure this is a bug, but the way that Discourse handles conversion from PNG to JPG images is not obvious. There are two settings that control it. To prevent PNGs from being converted, you need to set both the
png to jpg quality and
recompress original jpg quality settings to 100. One other thing to note is that PNG images will be automatically converted to JPGs if they are pasted into the composer. I don’t believe the latter issue will affect Imgur links, but it is possible that I am wrong about that.
It would be great if we could find a way of making it clear on the settings page how these settings can be used to prevent PNGs from being converted. If that can’t be done, we’ll need to properly document the functionality on Meta.