PrestaShop (starting from version 22.214.171.124) allows the application administrator to define the quality of the products’ images.
Step 1: The products images can be uploaded from your PrestaShop admin area -> Catalog.
Step 2: Pick a product and open the Images tab to browse and upload images.
Step 3: Once you are ready you can modify the images quality from the PrestaShop admin area -> Preferences -> Image.
Step 4: There you can set the JPEG and the PNG quality values from defined ranges. The worst quality of the graphics leads to shorter loading times for the images. Basically, if you manage a large online shop with many products you can consider decreasing the images quality in order to improve the whole web site performance.
Step 5: Once the values are defined click on the Regenerate thumbnails button. Allow some time for the procedure to be completed.