What is the easiest way to display product thumbnails on the category page instead of full images?

Question

WooCommerce

When you open a page for any product category on a WooCommerce powered site, by default the products are shown with their original images (large enough in size). Yes, they are resized along the grid, but the original ones are loaded anyway. Are there any easy ways (preferably without paid plugins) to make it so that instead of these large images, pre-prepared less voluminous product images are displayed?