This page can be accessed by going to System > Settings > SystemGo to System → Settings → System
Set .jpeg images quality level for new thumbnails generation. Best quality, biggest file. There is no single "best" JPEG compression setting, and depending on the type and complexity of image detail (or lack of complexity and detail), you may find yourself using 40-60, 70-80, or 90-100 as appropriate for the product photos.
...