Yes, with a RokBox, you need to use the Image type button. The image link in the parameters should just be the full path to your image. While you can use a relative path, using the full image path with domain is the best, like
yoursite.com/images/facebook-login.jpg
Mainly, just make sure you can go to the image link in your browser directly, and then use that URL in the image URL setting of the SCLogin module.
Hope that helps, but if you still run into issues, just let us know!
Thanks,
Alex