Caesium

Caesium is a simple program to compress any image on your hard drive so it takes up much less space, while only minimally reducing the quality in the process. Using this program is simple. You just have to drag the image or images (you can convert large batches of photos at a time) to the window, select the desired degree of compression by sliding the quality tab between 1 and 100, and lastly, click 'compress.' The more you compress photos, the less space they'll take up on the hard drive and the lower the quality. Thus it's best to try to hit a balance between high quality and a small space reduction, as if you reduce the size of a file by 90%, you probably won't like the effect on the photo quality.