You can activate the inventor tracking module. This shows how many products there are in stock, if set in the product details.
Also you could edit products.tpl And use an if statement to show an image when in stock. Also was thinking about this idea. Will try to work it out when I'm back at home.
Edit: Copied this code from the product.tpl page:
Code:
{if $config.Appearance.show_in_stock eq "Y" and $config.General.unlimited_products ne "Y" and $product.distribution eq ""}
<tr><td width="30%">{$lng.lbl_in_stock}</td><td nowrap="nowrap"><span id="product_avail_txt">{if $product.avail gt 0}{$lng.txt_items_available|substitute:"items":$product.avail}{else}{$lng.lbl_no_items_available}{/if}</span></td></tr>
{/if}
You can just remove the <td width="30%"> and add an image before the code so it will say how many items there are in stock, or if the product is out of stock. You can paste the code in your products.tpl anywhere you want.