Conditional code help need! – 2 thumbnails showing

FYI – I think I understand how to fix my problem but am only code at editing existing code and could really use some help writing this fix.

So here’s the problem… I now have 2 thumbnails showing on my single post page… why?

Because I’ve added the following code to my functions.php:

//check attachment
function checkimage($size=medium) {
	if ( $images = get_children(array(
		'post_parent' => get_the_ID(),
		'post_type' => 'attachment',
		'numberposts' => 1,
		'post_mime_type' => 'image',)))
	{
		foreach( $images as $image ) {
			$attachmentimage=wp_get_attachment_image( $image->ID, $size );
			return $attachmentimage;
		}
	}
}

//get attachment
function postimage($size=medium) {
	if ( $images = get_children(array(
		'post_parent' => get_the_ID(),
		'post_type' => 'attachment',
		'numberposts' => 1,
		'order' => 'ASC',
		'post_mime_type' => 'image',)))
	{
		foreach( $images as $image ) {
			$attachmentimage=wp_get_attachment_image( $image->ID, $size );
			echo $attachmentimage.apply_filters('the_title', $parent->post_title);
		}
	}
}

With corresponding code in my single.php file to show the image attached to each post:

<?php if(checkimage()) { // if there is an image ?>
<?php postimages('medium'); ?>
				<?php } ?>

(which have been uploaded via the default media gallery)

This all works great except for in the case when I need to use a custom field key to embed a thumbnail (which is necessary for one particular page due to the styling of those posts).

What I need now is some conditional code to fix the problem… I’m thinking it should be something like:

Checkimage
IF there is a thumbnail THEN post thumbnail and END
ELSE postimage

Does that sound right? and if so… what’s the code for that?