You're looking for do_shortcode()
:
<?php echo do_shortcode("[shortcode]"); ?>
So in your template, it will look as such:
<div id="main" class="large-12 medium-12 columns clearfix" role="main">
<h2 class="press-list-header">Articles</h2>
<ul class="press-list">
<?php echo do_shortcode("[picture image=\"http://example.com/image1.jpg\" thumbnail=\"http://example.com/image1-thumbnail.jpg\"]"); ?>
<?php echo do_shortcode("[picture image=\"http://example.com/image2.jpg\" thumbnail=\"http://example.com/image2-thumbnail.jpg\"]"); ?>
<?php echo do_shortcode("[picture image=\"http://example.com/image3.jpg\" thumbnail=\"http://example.com/image3-thumbnail.jpg\"]"); ?>
</ul>
</div>
Adding to your UPDATE, the do_shortcode
function is still the way to go:
<?php the_post(); global $post; echo do_shortcode($post->post_content); ?>