Where Did Post Thumbnails go??

I have looked around the forums and even tried a couple plugins, to no avail. I have extremely rudimentary web skills, so I struggle with figuring out how everything is connected and where to look for what needs to be fixed. I don’t post on my blog frequently, and my last post was about a year and a half ago (8/2012). It used to be that when I added a photo at the top of my post, that photo automatically became the thumbnail that accompanied the post on my front page, or if I posted a link on Facebook. Is this not a feature anymore? I am also not sure if it has something to do with my Theme (zeeBusinessPro), although I have added pages on that theme and they created their own thumbnails. I cannot find a “Featured Image” option anymore either, which I never used anyway. My site is newleafnaturalhome.com , and you can see previous posts at the bottom, and the newest one that’s missing its thumbnail. Ideas?