Hi Articulate team!
I have encountered the same problem and discovered this thread and the workaround (with _NOPROCESS_ in the file name), which works great, so many thanks for this!
However, I still think that this is a problem that should be fixed after all these years, rather than just providing a workaround (which most people will never see – I've been noticing the bad image quality for a while now before finally stumbling upon this thread).
The solution seems straightforward: don't change the compression/format of uploaded PNG images to JPEG (which is for photos and will always perform badly on illustrations, screenshots, etc. which is what PNG is normally used for). So, just keeping the format of the original file would solve the issue, while still allowing Rise to optimise the file size.
Hope to see this fixed soon and wish you all a nice day!