It should be there already by default, the simplest code to display it would be:
<img src="{$manufacturer.image_url|amp}" />
Of course you should probably do some checking to see if they have an image and add an alt tag:
Code:
{if $manufacturer.image ne ''}
<img src="{$manufacturer.image_url|amp}" alt="{$manufacturer.manufacturer|escape}" />
{/if}