Hey Chris,

I believe you probably have pagespeed installed and it is actually converting URL of images into base64 values. In order for Facebook to retrieve the image url, it needs to search for the og:image tag. If you view the source code of the page, you'll see that the og:image tag is being replaced with base64 values. I have a feeling that this is causing issues for Facebook.

There is also another possibility that the image size is too small. There is a minimum requirement for image sizes, https://developers.facebook.com/docs/sharing/best-practices