One simple way to do it:
1. Admin->Configuration->Product Listing->Products Quantity -- set a column number greater than 0
2. Alter /includes/modules/product_listing.php, and change:
Code:
        $lc_text = $listing->fields['products_quantity'];
to:
Code:
        $lc_text = ($listing->fields['products_quantity'] > 0 ? zen_image(DIR_WS_IMAGES . 'instock.jpg', 'In Stock', '60', '80', 'class="listingInStockImage"') : zen_image(DIR_WS_IMAGES . 'outofstock.jpg', 'Out Of Stock', '60', '80', 'class="listingOutOfStockImage"'));
Replace '60' with the appropriate width, and '80' with appropriate height, and create 2 new image files: /images/instock.jpg and /images/outofstock.jpg