List category posts plugin get_post_meta problem

Hi there
I’ve got a problem with displaying custom fields value
I use get_post_meta to retrive value of pic_tn field…
Any ideas how to solve this problem?

Thanks up front!
The code:

foreach($catposts as $single):
  $output .= "<div class=\"featured\"><div class=\"boxgrid caption\"><a alt=\"".$single->post_title."\" href=\"".get_permalink($single->ID)."\"><img style=\"\" alt=\"sztaa - ".$single->post_title."\" src=\"".get_post_meta($post->ID,"\"pic_tn\"")."\"/></a><div class=\"cover boxcaption\"><h2><a title=\"".$single->post_title."\" alt=\"".$single->post_title."\" href=\"".get_permalink($single->ID)."\">".$single->post_title."</a></h2><!--<p><a rel=\"nofollow\" href=\"".get_permalink($single->ID)."\">more...</a></p>--></div></div></div>";