First of all enable "Custom Fields" under "Screen Options".
under "Name" put "thumb" and under "Value" insert link to your 500px picture.
Next you need to edit your index.php file and add this:
Code:
<?php if( get_post_meta($post->ID, "thumb", true) ): ?>
<p><a href="<?php the_permalink() ?>" title="<?php the_title(); ?>">
<img src="<?php $values = get_post_custom_values("thumb"); echo $values[0]; ?>" height="120" width="160" alt="<?php the_title(); ?>" /></a></p>