There are several settings which seem relevant:
recompress original jpg quality(“Quality of uploaded image files (1 is lowest quality, 99 is best quality, 100 to disable).”)png to jpg quality(“Quality of the converted JPG file (1 is lowest quality, 99 is best quality, 100 to disable).”)composer media optimization image encode quality(“JPEG encode quality used in the re-encode process.”)
I am not sure how the last one relates to the first two — or what happens to webp or other file types that aren’t PNG or JPEG.
(Also, note for any Discourse UX people reading this: inconsistent use of JPEG and JPG makes this harder to find.)