Mac users: reduce image file sizes a lot, very easily

Open a folder with some images/photos. View the folder in list mode. Open one of the images/photos up in preview. Keep the folder behind visible. Goto Tools select adjust color. Make a change, any change, small or big. I usually click auto levels. watch the file size as you hit apple Apple S. 

Magically reduce file sizes on average, 30-50%, Same dimensions, same resolution. Good for trimming fat off your site, fast. Reduce bandwidth.