One quick way that comes to mind is to implement a counter (of course there may be many other ways):
<?php $images = get_field(‘image_gallery’);
if( $images ) {
$counter = 0;
?>
<div id=”imagegallery”>
<?php
foreach( $images as $image ) {
if ( $counter == 0 ) {
echo 'first image';
else {
echo 'all other images';
}
$counter++;
}
?>
</div>
<?php } ?>
