1) You can add your own images to that the social network subdirectories in that folder and they'll automatically be detected. You can then select your default image in the admin area. If you're using our tag system to add the login buttons to your page, you can also set which image to use, like:
{JFBCLogin providers=facebook,twitter,linkedin image=icon_with_custom_text.jpg}
That tag will show only the Facebook, Twitter and LinkedIn logins (even if you have others configured) and will show the icon_with_custom_text.jpg image from the folder below:
/media/sourcecoast/images/provider/<social_network>/
So, you can have different 'sets' of icons to use how you wish.
2) To overwrite the socialstream data, please edit the themes files in the /media/sourcecoast/themes/scsocialstream directory or use template overrides in the
/templates/<YOUR_TEMPLATE>/html/com_jfbconnect/themes/scsocialstream/default/
directory
I hope that helps!
Alex