to hide the login just command the line
//echo elgg_view_form('login', array('id' => 'facebook-header-login'));
and to add image i.e a banner in the header, you can write like this also
echo "<img src=\"{$vars['url']}mod/plugin_name/graphics/banner.png\" border=\"0\" />";
or else you can even try out in this way by closing the php tag of above line using ?>
and write as
<img src="<?php echo $vars[url]; ?>mod/plugin_name/graphics/banner.png">
Am not sure on this.. but this may help you... you can write either directly in php or inserting php in html tag too...
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