Resizing Doesn't Work with Older Images

I noticed that my older images, older than October 2013, the resizing feature doesn’t work. At first I thought it was a caching issue or a custom image size issue, but after changing to the WordPress ‘Thumbnail’ size and disabling my caching plugin, it still doesn’t work.

By trial and error, I used different images and found out the images from present to October 2013 work fine. FWIW, this is what I’m using on my home page (in part):

<?php foreach ( get_terms( 'portfolio', array( 'include' => '839' ) ) as $cat ) : //Landscape?>

<div class="front-page-item">
	<figure itemprop="image" itemscope itemtype="https://schema.org/ImageObject">
		<h3><?php echo $cat->name; ?></h3>
		<div itemprop="image" class="front-page-image">
			<a href="<?php echo get_term_link( $cat->slug, 'portfolio' ); ?>"><img src="<?php echo z_taxonomy_image_url( $cat->term_id, 'front-page' ); ?>" width="495" height="330" alt="<?php echo $cat->name; ?>" title="<?php echo $cat->name; ?>" /></a>
		</div>
		<figcaption itemprop="description"><?php echo $cat->description; ?></figcaption>
	</figure>
</div>	

<?php endforeach; ?>

I’m going to guess it has something to do with an older version of MySQL/PHP?

To add, the code above works for actually resizing, but the file size doesn’t change.