Scott,
Very glad to hear 5.1.0 is doing well for you. There was a ton of effort put into it to make sure it improved usability while adding some awesome new features... It's been a very smooth release so far, and we're very excited to gear up the promotion engine to make sure every knows its out next week. With that said, there was a v5.1.1 release earlier today with some minor bug fixes.. just so you know.
For the image button, we've tried to make it super simple. Just use the HTML below:
<a href="javascript:void(0)" onclick="jfbc.login.facebook();"><img src="/your-image-here.jpg" /></a>
When you're ready for a Google button, just update jfbc.login.facebook() to jfbc.login.google. When Twitter comes out, I'm sure you can imagine what the update should be then too
Let me know if that helps or you run into any issues.
Thanks,
Alex