Then you can simply edit menu.tpl. Be careful here though, as that template is shared between the admin and customer sides. To have something only show for the customer, use this:
{if $usertype eq "C"}
show image
{/if}
To call an image in X-Cart, use this:
<img src="{$ImagesDir}/imagename.jpg" alt=""/>
Upload the image to the skin1/images directory.