It’s worth saying that the current default to optimized JPEG is not a misguided decision! I love PNG 
and abhor JPG 
for graphics but for this issue I totally support the current regime (both meanings: authority and system).
The Discourse team have made a realistic assessment of what will make Discourse successful out of the box:
-
The average site owner-administrator will not be looking for image quality options. They don’t have image quality as a goal and they will not be consider the default image file format to be an issue. So defaulting to optimised JPG produces the least grief and the greatest benefit for the most people.
-
PNG-ophiles like myself are always looking for site options to preserve image quality and information. Likewise photo- and image-driven sites will have image quality as a central decision.
A practical consequence of the current approach is likely to be a seamless transition to lossy WebP. Then the issue of lossy vs lossless WebP will probably arise in further topics here. 
If you’ve ever optimized a lot of PNG files then you’d know that the likely outcome is unattractive. PNG optimizers take forever compared to JPG optimizers. It also takes more smarts to create PNG images that can be most effectively optimized, e.g. https://medium.com/@duhroach/reducing-png-file-size-8473480d0476 or How To Optimize PNG — Smashing Magazine