To use PHP in your templates, this:
<? include(this.php); ?>
should be formatted like this:
<?php
include("<full server path to included file>/this.php");
?>
I have found this syntax to be more 4images template parsing friendly although I have used the other syntax successfully in some places. Your results may vary.
Also, I too would like to know how to add a new, custom template. I think part of the answer is to add
?template=MyCustomTemplate
to the URL but I'm not sure if the template has to somehow be registered with 4images.