Hi, Arasan. This is not achievable with fieldsMapping only.
The JSON you specified is what comes back in Facebook response for “picture” field. You can check it with Facebook Graph API Explorer tool. So extracting pure picture URL is up to you. You can try to achieve this with event handler for Facebook registration in Backendless Custom Business Logic.
At the moment there is no event handler for RegisterWithFacebook (just a plain “login/password” register event handler), but we’ll add it in upcoming release, and notify you on this.