JFBConnect only ships with those 2 images, but you're free to use any that you want. We have a simple system in place that lets you add whatever images you want. To use different images, simply upload them to the /media/sourcecoast/images/provider/xyz directory (where xyz is the specific social network you want add images for). Once you add those there, you should see them as available to use in the JFBConnect configuration area when selecting the default image.
If you forget, you can hover over the "Default Login Button" label in the configuration area. The tooltip there will provide the proper path.
I hope that helps, but if you have any other questions or suggestions, just let us know.
Thanks,
Alex