JFBConnect has a native way to use your own custom buttons out of the box without any template overrides. For details, please see:
www.sourcecoast.com/jfbconnect/docs/social-login/login-button
Mainly, copy the custom login button image to the correct social network directory in:
/media/sourcecoast/images/provider
Then, update the "Social Button Settings" in the SCLogin module.
I hope that helps, but if you need anything else, just let us know.
Thanks,
Alex