WordPress grab first image from gallery

I had the same issue and was previously using get_children() to retrieve the top 4 images from a gallery for a given page.

Here’s the solution I came up with:

// helper function to return first regex match
function get_match( $regex, $content ) {
    preg_match($regex, $content, $matches);
    return $matches[1];
} 

// Extract the shortcode arguments from the $page or $post
$shortcode_args = shortcode_parse_atts(get_match('/\[gallery\s(.*)\]/isU', $post->post_content));

// get the attachments specified in the "ids" shortcode argument
$attachments = get_posts(
    array(
        'include' => $shortcode_args["ids"], 
        'post_status' => 'inherit', 
        'post_type' => 'attachment', 
        'post_mime_type' => 'image', 
        'order' => 'menu_order ID', 
        'orderby' => 'post__in', //this forces the order to be based on the order of the "include" param
        'numberposts' => 1,
    )
);

Now, 'numberposts' => 1 above doesn’t seem to work when you specify more than one id in the include parameter. So, you have a couple of choices:

1. Explode the “ids” argument and grab only the first ID for use in your get_posts call
2. Grab only the first attachment returned from get_posts (but you’ll still be retrieving all attachments in the gallery). Not ideal, but will work.

Hope that helps!