Eros,
The image button is a feature of newer SCLogin module versions. You'd need to upgrade if you want to use that functionality from the easy parameters.
If not, you'll need to create your own custom image button instead of the Login with Facebook in the SCLogin module. You can see how in our
Common Joomla Facebook Integration Questions
guide. You can add that custom HTML anywhere on your site or in a custom HTML module.
Hope that helps answer your questions,
Alex