In the past few weeks, I had contact with a very interesting tool for Sitecore, It’s called Dianoga. The tool basically compresses your image after being requested from the server. The Dianoga saves this compressed image in App_Data/MediaCache (your original Image in Media Library stays intact. ) and Sitecore returns to the client the compressed image. I found this tool very helpful and the tool provides information regarding the images that were compressed and writes this information on Sitecore logs.
The last version it’s the 3.1.1 and requires .NET Framework 4.5.2, I recommend the latest version since it solves an important bug as describe on the changelog
Sitecore Tips 