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When running a website through Google PageSpeed Insights I get the following response:
Properly formatting and compressing images can save many bytes of data.
Optimize the following images to reduce their size by XX.XKiB (X% reduction).
Losslessly compressing http://www.website.com/path/to/image/image_file.jpg could save XX.XKiB (XX% reduction).
What do the tool mean by "Losslessly compressing?" Sounds like I could reduce the filesize without sacrificing image quality; but how?