User Submitted Posts Plugin

keep in mind, if users can add more than one image it will appear also, not sure how that will work for your layout.

you can limit the number of images with this function:

function get_random_gallery_images(){
	global $wpdb,$post;
		$ids = "";
		$counter = 0;
		$number_of_posts = 1;
		$args = array(
		'post_type' => 'attachment',
		'numberposts' => 1,
		'post_status' => null,
		'orderby' => 'rand',
		'post_parent' => $post->ID
		);
		$attachments = get_posts($args);
		if ($attachments) {
			foreach ($attachments as $attachment) {

				if ($counter != 0) {
					$ids .= ','.$attachment->ID;
				}
				else {
					$ids .= $attachment->ID;
				}
				$counter++;
			}
		}
		return $ids;
}

output code:

<?php
$attachment_ids = get_random_gallery_images();
echo do_shortcode('[gallery columns="3" include="'.$attachment_ids.'" link="file"]');
?>

well, guess we’re resolved. carry on.

ah shucks