Figures it out. It was the spaces in my_acf_admin_head. Cleaned it up like this…
<?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'); ?>
