<?php
exec
("/usr/local/bin/composite -watermark 30% -gravity center piglet.gif earth.jpg watermark/watermark_logo.jpg");
?>

This breaks down into:


An image watermarked with another image

Multiple watermarks

<?php
exec
("/usr/local/bin/composite -watermark 30% -tile piglet.gif earth.jpg watermark_logo1.jpg");
?>

Add multiple image watermarks
<?php
exec
("/usr/local/bin/composite -dissolve 30% -tile piglet.gif earth.jpg watermark_logo2.jpg");
?>

Add multiple image watermarks