WordPress helper function that retrieves the first image from a post, with basic support for swapping the image size.

get-first-image.php

<?php

/*

Based on the original CSS-Tricks code (http://css-tricks.com/snippets/wordpress/get-the-first-image-from-a-post/)

## Usage
Include in functions.php

<?php echo get_first_image('thumbnail'); ?>

*/

function get_first_image($size = false) {
    
    global $post, $_wp_additional_image_sizes;
    $first_img = '';

    $output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
    $first_img = $matches[1][0];

    if (empty($first_img)) {
        return;
    }

    if ($size && $_wp_additional_image_sizes[$size]['crop'] == 1) {
        $size = '-' . $_wp_additional_image_sizes[$size]['width'] . 'x' . $_wp_additional_image_sizes[$size]['height'] . '.jpg';
        $pattern = '/-\d+x\d+\.jpg$/i';
        $first_img = preg_replace($pattern, $size, $first_img);
    }

    return $first_img;
    
}