// Search for the following line of code in the portfolio.php file:

<a href="<?php echo get_post_meta($post->ID, "full_image_value", $single = true); ?>" rel="prettyPhoto[gallery]"><img src="<?php echo get_post_meta($post->ID, "post_image_value", $single = true); ?>" alt="<?php the_title(); ?>" /></a>

// Change it to this:

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