I'm wandering how and when the JPEG quality ratio (75% by default in admin/config/media/image-toolkit) is applied to the images. 1/ I suppose it is used a first time when the image is upload and scaled to feat the max width/height set up in the content type field. So, in case the original uploaded image already feats the max width/height, is it processed and compressed anyway or not? 2/ Then, I suppose this ratio is applied a second time when generating image styles at page load. So this means that the resulting displayed image has been compressed two times with 75% quality loss (resulting ±56%). Is that it? Thanks for your help.
I tested it on a local server. Now I can say that: 1/ Image is processed at upload ONLY if it is larger than maximum authorized width/height (if it fits in this max size, it is not processed). In this process, the JPEG quality ratio set up in configuration (admin/config/media/image-toolkit) will be applied. This first step generates the "drupal original image" with will be used to generate image styles. 2/ In this new process of generating the image styles, the JPEG quality ratio will be applied AGAIN when resizing the thumbnails.
This means that it seems better to keep the JPEG quality ration roughly around 60-80% to have lights pages, but in order to avoid it to be applied twice on the images styles thumbnails displayed in the pages, upload "original" images that are 100% quality JPEG and fit to the max allowed size (so that are not processed at upload).