Warning: imagejpeg()

hi..
After install plugins ..
I created template page to test..
in page I wrote following code to retrieve all posts with feature image,

$args = array(
			'post_type'      => 'post',
			'posts_per_page' => -1
			);
			$rand_posts = get_posts( $args );
			foreach ( $rand_posts as $post ) {
			$thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'full' );
			echo $url = $thumb['0']; ?>
			<img src='<?php thumbGen($url,171,56,"effect=grayscale&halign=left&valign=top"); ?>' alt='' />
			<?php }

i got error.every time in foreach.here it is…

Warning: imagejpeg() [function.imagejpeg]: Unable to open 'C:/wamp/www/C:\wamp\www\testing_cart\wordpress/wp-content/thumbgen_cache/ddd13e6ff4bebb83bf2dbd2577b99c1d.jpg' for writing: Invalid argument in C:\wamp\www\testing_cart\wordpress\wp-content\plugins\thumbgen\thumbGen.php on line 159

https://www.ads-software.com/plugins/thumbgen/