When the JPG image quality isn't specified, the default quality being used is 90 (90%). If the original JPEG quality is less than 90% and no quality is specified, we don't change the JPEG quality.
You can manually set the
quality parameter (
q in URLs) to the value your require (1-100).
For example, the following sets the JPG quality of the image to 70% of the original image's quality: