If I understand your question correctly, I believe this is what you’re looking for:
<div class="postthumbnail">
<?php
$usercrop = get_field('usercrop');
if ($usercrop) {
$image = wp_get_attachment_image_src(get_field('usercrop'), 'full'); ?>
<img class="post-thumbnail img-responsive postpic" src="https://wordpress.stackexchange.com/questions/232716/<?php echo $image[0]; ?>" alt="<?php echo get_the_title(get_field('usercrop')) ?>" />
<?php }
?>
</div>
It checks the field and only displays the HTML if get_field('usercrop')
returns something