Tweaking the dp_attachment function

Help please!

On my blog, http://www.incredibledad.com, the theme I’m using has the following code to get thumbnail images for summary of posts in the middle of the home page.

# Displays post image attachment (sizes: thumbnail, medium, full)
function dp_attachment_image($postid=0, $size='thumbnail', $attributes='') {
	if ($postid<1) $postid = get_the_ID();
	if ($images = get_children(array(
		'post_parent' => $postid,
		'post_type' => 'attachment',
		'numberposts' => 1,
		'post_mime_type' => 'image',)))
		foreach($images as $image) {
			$attachment=wp_get_attachment_image_src($image->ID, $size);
			?><img src="<?php echo $attachment[0]; ?>" <?php echo $attributes; ?> /><?php
		}
}

Unfortunately, instead of picking up the first image in the post, it’s picking up the second image in the post (unless the first image is the only image).

2 questions please:

1) Is there a way I can force it to always pick up the first image?
2) Is there a way to perhaps use a custom field to force an image?

Many thanks in advance!

ID