If Product image is not showing on facebook page, in this issue you need to create an app for Sharing.
Click here for more info and get you facebook app id
Then add below code to website header
<meta property=”fb:app_id” content=”your-app-id”/>
note: dont keep spaces in image name and folders.
You need to crate an app for sharing
Test here : https://developers.facebook.com/tools/debug/sharing/