I use the following on client sites:
HTML
<div align="center" style="width: 100%;">
<img src="/forum/images/logo.jpg" alt="" style="border:0;height:auto;max-width:100%;">
</div>

As for using PHP in the includes boxes, you have to use full PHP in them, and only some of them allow it, so example:
Code
<?php echo($config["BASE_URL"]); ?>

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