skin1/customer/main/product.tpl
Code:
{if $product.avail eq "0"}
<img src="{$ImagesDir}/out_of_stock.jpg" />
{elseif $product.avail gt "0" && lt "10"}
<img src="{$ImagesDir}/low_stock.jpg" />
{elseif $product.avail gte "10"}
<img src="{$ImagesDir}/in_stock.jpg" />
{/if}