PHP if if else endif和wordpress的循环

I've a problem with the loop in wordpress, my website is a feed aggregator and basically I want my website to display an image if the feed contains an image or a specified image if it doesn't. here's the code I actually use:

    <?php $enclosure =  get_post_meta($post->ID , 'enclosure', $single = true); ?>
    <?php $image=explode(chr(10),$enclosure); ?>
    <?php if(!is_null($image)) : ?>
    <div class="left">
    <a href="<?php the_permalink() ?>" rel="bookmark" title="Link all'articolo: <?php     the_title_attribute(); ?>"><img src="<?php echo $image[0]; ?>" alt="<?php the_title_attribute(); ?>" style="width:150px; text-align:center;"/></a>
    </div>
    <?php endif; ?>

It works very well but now I want to show a "default" image if the feed doesn't provide one, I tried in this way:

    <?php $enclosure =  get_post_meta($post->ID , 'enclosure', $single = true); ?>
    <?php $image=explode(chr(10),$enclosure); ?>
    <?php if(!is_null($image)) : ?>
    <div class="left">
    <a href="<?php the_permalink() ?>" rel="bookmark" title="Link all'articolo: <?php     the_title_attribute(); ?>"><img src="<?php echo $image[0]; ?>" alt="<?php the_title_attribute(); ?>" style="width:150px; text-align:center;"/></a>
    </div>
    <?php else : ?>
    <div class="left">
    <a href="<?php the_permalink() ?>" rel="bookmark" title="Link all'articolo: <?php     the_title_attribute(); ?>"><img src="PATH TO THE DEFAULT IMAGE" alt="<?php the_title_attribute(); ?>" style="width:150px; text-align:center;"/></a>
    </div>
    <?php endif; ?>

But it doesn't work... Where am I wrong? Sorry for my english!

Thanks!

try to use

if($image=="" && $image=="null")
{

}

try

if(file_exists("file path")){

}

instead of

`if(!is_null($image)) :`