Try using this extension to override the "Accept" request header value for the site so that it's text/html,application/xhtml+xml,application/xml;q=0.9,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9 instead of text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9.
Notice image/webp missing in the former. This stops Opera from telling the site that it supports image/webp so that the site falls back to jpeg.
You might have to clear your cache first to get the updated result for the test pic you linked to.
