In typical fashion, I got there in the end! This is what I ended up using in my template files:
<?php $image = get_field('field_name'); ?>
<img src="<?php echo get_template_directory_uri(); ?>/images/loader.gif" alt="" data-src="<?php $image = get_field('field_name'); echo($image['sizes']['custom-size']); ?>" data-src-retina="<?php $image = get_field('field_name'); echo($image['sizes']['custom-size-retina']); ?>" />
And to the functions.php with these declarations:
add_image_size( 'custom-size', 300, 300, true ); // Standard pixel density
add_image_size( 'custom-size-retina', 600, 600, true ); // Double pixel density
