Gotcha. Yes, that should be pretty easy if you already have the image you want to use.
First, create a template override for the SCLogin module's login view. Do that by copying /modules/mod_sclogin/tmpl/login.php to /templates/<YOUR_TEMPLATE>/html/mod_sclogin/login.php (creating any directories that don't already exist).
Then, edit that login.php file you just made. Look for the following (around line 107):
if ($params->get('showRegisterLink'))
echo '<li><a href="' . $registerLink . '">' . JText::_('MOD_SCLOGIN_REGISTER_FOR_THIS_SITE') . '</a></li>';
and change to:
if ($params->get('showRegisterLink'))
echo '<li><a href="' . $registerLink . '"><img src="/link-to-your-image.jpg" /></a></li>';
Let us know if you run into any trouble, or have questions!
Thanks,
Alex