@{
    var _size = '';
    if (imageSize === 'custom' && customImageSize) {
        _size = customImageSize;
    } else if (imageSize && imageSize !== 'custom') {
        _size = imageSize;
    }
}
<?php echo theme_get_post_thumbnail(array(@if (_size) {'size' => '@_size', }'imageClass' => '@imageStyle.classNames', 'class' => '@classNames')); ?>