I've compress all the images with WP Smush plugin. when i test my site in webpagetest then there i see N/A Compress Images. I'm also using w3c cache plugin. You can checked the code in attached file. I also using MaxCDN
N/A just means it didn't find any images on the page that it could check. If the page is HTTPS and the test was done with Chrome then that would explain it (right now it doesn't have access to the raw responses in Chrome for HTTPS content). The other thing that may be is if all of the images are PNG's because WPT only checks JPEG images for lossy re-compression.