Not Ray but this would be the easiest way to achieve what you want:
<div class="galleryContainer">
<?php $count = 0; // add count and set to zero ?>
<?php
while(!$Slideshow->atEnd()) {
?>
<a href="#">
<picture class="responsive-picture <?php echo($count === 0)?'mainGalleryImage'?'thumbnailGalleryIMage'; // check count value and display appropriate class ?>"><img alt="" src="img/picture-12.svg"></picture>
</a>
<?php $count++; // increment count at end of each loop ?>
<?php
$Slideshow->moveNext();
}
$Slideshow->moveFirst(); //return RS to first record
?>
</div>
You simply add a count and then check it's value and display the appropriate css class.
Regards,
Dave Buchholz
I-CRE8 Limited