For now, you'll have to edit the getLoginButton() function in the /components/com_jfbconnect/libraries/provider/facebook.php file.
In the 6.1 release of JFBConnect (due in late June), there will be a new option to select the button you want to use. All you'll need to do is put the image in the /media/sourcecoast/images/provider folder and JFBConnect will automatically detect and let you use any png, jpg or gif files in those folders.
I hope that helps answer your question, but if you need more assistance, just let me know.
Thanks,
Alex