Do it the same way as you would with normal html and work the smarty bits around it.
ie.
{* $Id: categories.tpl,v 1.16 2002/10/21 07:06:43 zorg Exp $ *}
<TABLE border=0 cellPadding=5 cellSpacing=0 width=100%>
<TR>
<TD class=VertMenuTitle height=26 width=26 valign=center>[img]{$ImagesDir}/dingbats_categorie.gif[/img]</TD>
<TD class=VertMenuTitle height=26 valign=center width="80%"><FONT class=VertMenuTitle>{$lng.lbl_categories}</FONT></TD>
</TR>
<tr><td colspan=2 nowrap>
{if $config.General.root_categories eq "Y"}
{section name=cat_num loop=$categories}
<font class=CategoriesList>
[img]./yourimages/yourimage{ $categories[cat_num].categoryid }.gif[/img]</font>
{/section}
{else} {section name=cat_num loop=$subcategories}
<font class=CategoriesList>
{ $subcategories[cat_num].category|escape }</font>
{/section}
{/if}
</td></tr>
</TABLE>
Where each image here is called yourimage{category number}.gif