Need help for query / php

Hi guys

i have a problem in buliding query

problem :it shows post image and default image

i want to show post image if not available, default image
i have placed the code and function for thumbnail which i used

code

<div class="newssection">
					<div class="heading">
						<h3>Latest Posts in <a href="'.$cat_link.'" title="'.$cat_name.'">'.$cat_name.'</a> category</h3>
						<div class="clear"></div>
					</div>
							global $post;
							$myposts = get_posts("numberposts=1&category=$cat_number");
							foreach($myposts as $post)
								{
									$posttitle = get_the_title();
									$posttime = get_the_time('F jS, Y');
									$postauthor = get_the_author();
									$postlink = get_permalink();
									$text = get_the_excerpt();
									$text = substr($text , 0, 150);
									$nothumb = get_bloginfo('template_directory');
									 $thumb = dp_attachment_image($post->ID, "Thumbnail", true);  

									echo '<div class="article">';
										echo '<div class="left">';
											echo '<img src="';
												if ($thumb == NULL) { echo ''.$nothumb.'/images/defaultthumb.jpg'; } else { echo $thumb; }
											echo '" alt="'.$posttitle.'" />';
											echo $posttime.'<br />';
											echo $postauthor;
										echo '</div>';

function for thumb

# 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
		}
}