Product Images – High-Resolution Square Images on Product List Pages

Looking at Mage_Catalog_Helper_Image->resize

/**
 * Schedule resize of the image
 * $width *or* $height can be null - in this case, lacking dimension will be calculated.
 *
 * @see Mage_Catalog_Model_Product_Image
 * @param int $width
 * @param int $height
 * @return Mage_Catalog_Helper_Image
 */
public function resize($width, $height = null)
{
    $this->_getModel()->setWidth($width)->setHeight($height);
    $this->_scheduleResize = true;
    return $this;
}

So resize by ->keepFrame(true)->resize(null, 300)

Category loading has changed in 2.1, so this may only be relevant from 2.1 onwards:

The image gallery is added to the product via an extension interface defined via di.xml. The upshot is that we can manually create an instance of the gallery ReadHandler class and pass a product to load all its gallery images.

As usual in Magento 2 the best way to instantiate a class is via the __construct() method, so here is a stub block class:

use Magento\Catalog\Model\Product\Gallery\ReadHandler as GalleryReadHandler;

class Gallery
{
    protected $galleryReadHandler;

    public function __construct(
        GalleryReadHandler $galleryReadHandler
    )
    {
        $this->galleryReadHandler = $galleryReadHandler;
    }

    /** Add image gallery to $product */
    public function addGallery($product)
    {
        $this->galleryReadHandler->execute($product);
    }
}

In your template, assuming you have $product loaded via a product collection, you would be able to call:

$block->addGallery($product);
$images = $product->getMediaGalleryImages();
foreach ($images as $image) {
    ...
}