Seems I was mistaken, a little sleep can be really helpful. You can in fact do the following:
<?php
//create collection of all images first
$images = new Collection();
foreach ($screenshots as $project) {
foreach ($project->images() as $i) {
$images->data[] = $i;
}
}
$images = $images->paginate(4);
$pagination = $images->pagination();
foreach($images) as $image) ; ?>
<a href="<?php echo $image->url(); ?>">
<?php echo thumb($image, array('width' => 320, 'quality' => 70)); ?>
</a>
<?php endforeach ?>