Yes, since you want to put this on the front of the site the hook you should be using is wp_head
<!--USER INPUT STYLES FUNCTION-->
<?php function my_acf_admin_head() {
$backgroundimage = get_field('background_image', 'option'); ?>
<!--BEGIN USER INPUT STYLES-->
<style type="text/css">
/**Adds custom global background image**/
.main-content {
background-image: url ('<?php echo $backgroundimage; ?>');
}
</style>
<?php
}
add_action('wp_head', 'my_acf_admin_head');
?>
<!--END USER INPUT STYLES-->