I'm not very familiar with Facebook Opengraph tag. However, in order to use Optimizely, you'll have to place the code snippet on the page where the UI change occurs. Where does the UI change occur in this instance? Do you own the domain or does Facebook own the domain?
This will be really hard, because what Facebook does is that they cache the page and the open graph data for a while when you submit if for the the first time if you share/ like or commen on it. That makes it really hard to A/B test the images on there. You could still give it a try, the code for this will have to look something like: