Why no leverage browser caching?

I’ve installed WP Super Cache on my website so I could use a CDN to speed up my images. The plugin and CDN seems to be working just fine except for one thing: when I used GTMetrix.com to test my website’s speed, I got a score of “F” for leverage browser caching. The GTMetrix test says “expiration not specified” for my website’s CDN-processed images. Per the WP Super Cache instructions, I have set the Cache Timeout for 3600 seconds, and I also set the timer for 3600 seconds. What am I doing wrong? How do I fix this leverage-browser-caching problem? Thanks!