Thanks! I’ve ended up using this in order to still have a fallback image using Get The Image:
<?php $attachment_id = get_field('product_image');
$size = "thumbnail";
$image = wp_get_attachment_image_src( $attachment_id, $size );
if ( $image != 0 ) { ?>
<img src="<?php echo $image[0]; ?>" />
<?php } else {
get_the_image( array( 'size' => 'thumbnail', 'default_image' => get_stylesheet_directory_uri() . '/images/placeholder.jpg' ) );
} ?>