Drupal - Why is there quality loss on my image styles?

Under admin/config/media/image-toolkit there is an option to set the JPEG quality. By default this is 75%, so you should be able to up that to whatever you want.

You should use the following module : Image style Quality

Set JPG quality per image style This module allows you to specify a custom quality on different image styles you create. After installing and enabling this module, you will have the option to add a new effect to your image styles, enabling you to change the quality.

Uses and Application Having high quality images is great and improves the visual quality of your site. Problems can arise where you have dynamic content such as a page background images that users can upload. Having high image quality can lead to 2mb+ download sizes when using full 1920x1024 image styles. With this module, you can scale the quality of your large images down and leave your smaller thumbnails at your defined default.