vikkineal, here is what you need
<?php
// if plugin exists
if(class_exists('kdMultipleFeaturedImages')):
// insert entire image
kd_mfi_the_featured_image('featured-image-2', 'post');
endif;
?>
or alternatively you can call just the image like you wanted
<?php
// if plugin exists
if(class_exists('kdMultipleFeaturedImages')):
// insert entire image
?>
<img src="<?php echo kd_mfi_get_featured_image_url( 'featured-image-2', 'post', 'full' ); ?>" />
<?php
endif;
?>