You can start to get an idea of what to do
here
And here is the code I use on my site

you can click the www button below to see the result)
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Code:
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<table width="128" border="0" cellspacing="2" cellpadding="0" style="BORDER-COLLAPSE: collapse">
<tr><td>Navigate</td></tr></table><table width="128" border="0" cellspacing="2" cellpadding="0" style="BORDER-COLLAPSE: collapse">
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_house1.gif[/img]</td>
<td>Home</td>
</tr>
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_links.gif[/img]</td>
<td>Downloads</td>
</tr>
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_house.gif[/img]</td>
<td>Forums</td>
</tr>
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_gallery.gif[/img]</td>
<td>Gallery</td>
</tr>
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_gallery.gif[/img]</td>
<td>Google Flash</td>
</tr>
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_members.gif[/img]</td>
<td>GuestBook</td>
</tr>
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_members.gif[/img]</td>
<td>Members List</td>
</tr>
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_links.gif[/img]</td>
<td>Open Directory</td>
</tr>
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_chat.gif[/img]</td>
<td>Private Messages</td>
</tr>
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_smilies.gif[/img]</td>
<td>Recommend Us</td>
</tr>
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_stats.gif[/img]</td>
<td>Statistics</td>
</tr>
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_register.gif[/img]</td>
<td>Submit News</td>
</tr>
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_profile.gif[/img]</td>
<td>YourAccount
</td>
</tr>
</table>
<table width="128"><tr><hr width="100%" color="#5B5B5B">
</tr></table>
<table width="128" border="0" cellspacing="2" cellpadding="0" style="BORDER-COLLAPSE: collapse">
<tr><td width="20">[img]images/mini_icons_fiblack/new.gif[/img]</td><td><left>New</left></td></tr></table>
<table>
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_rules.gif[/img]</td>
<td>I-Frame Calendar</td>
</tr>
<td>[img]images/mini_icons_fiblack/icon_mini_rules.gif[/img]</td>
<td>I-Frame ShoutBox</td>
</tr>
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_calendar.gif[/img]</td>
<td>Nuke theme packs</td>
</tr>
<tr>
<td>[img]images/mini_icons_fiblack/icon_mini_gallery.gif[/img]</td>
<td>Desktop Wallpaper</td>
</tr>
<td>[img]images/mini_icons_fiblack/icon_mini_rules.gif[/img]</td>
<td>Comming Soon</td>
</tr>
</table> |
Since I started out with fiblack as a theme I just kept the fiblack folder in my images folder and replaced the mini-icons with charcoal ones, but you can put it anywhere and use whatever images you want as long as you change the path in the code. The horizontal line you want in <hr>. Find it in the code.
Oh yes last but not least, put this code in a new block that you make in admin/blocls and title it Main menu, and then de-activate your original mainmenu block. I hope this helps.