Using the value returned from an image object is covered on this documentation page http://www.advancedcustomfields.com/resources/image/
Create a URL and an image field, I’ll call them image and url
<?php
$image = get_field('image');
if (!empty($image)) {
$url = get_field('url');
?>
<a href="<?php echo $url; ?>" target="_blank"><img src="<?php
echo $image['url']; ?>" alt="<?php
echo $image['alt']; ?>" /></a>
<?php
}
?>
