<?php if ( get_post_meta($post->ID, 'thumb', true) ) { ?> <!-- DISPLAYS THE IMAGE URL SPECIFIED IN THE CUSTOM FIELD -->  

                  

                <div class="thumbnail"><img src="<?php echo get_post_meta($post->ID, "thumb", $single = true); ?>"  alt="<?php the_title(); ?>"  />        </div>      

                  

            <?php } else { ?> <!-- DISPLAY THE DEFAULT IMAGE, IF CUSTOM FIELD HAS NOT BEEN COMPLETED -->  

                  

                <div class="thumbnail"><img src="<?php bloginfo('template_directory'); ?>/images/no-img-thumb.jpg"  alt="<?php the_title(); ?>"  /> </div>  

                <?php } ?>