Kraken module allows the flexibility to control the quality of the optimized image.
You can control the optimized image quality by changing the following settings in the advanced configuration settings for Kraken :
"lossy" : "true/false"
Enable disable lossy compression.
"quality" : 1 - 100
This options will only be used if the LOSSY option is set to true. Can be a number in the range of 1 - 100. If the quality option is not provided it will default to “auto” where Kraken will optimize the images based on their characteristics.