Unless you're selling a lot of ads that need rotated, I'd just insert them using a text widget.
You could use code something like this:
<table width="100%" border="0" cellspacing="0" cellpadding="5">
<tr>
<td align="center"><a href="#"><img src="full-file-url" /></a></td>
<td align="center"><a href="#"><img src="full-file-url" /></a></td>
</tr>
<tr>
<td align="center"><a href="#"><img src="full-file-url" /></a></td>
<td align="center"><a href="#"><img src="full-file-url" /></a></td>
</tr>
</table>
Just replace the 'a href' with the link for the ad and the 'full-file-url' with the full URL to your ad image you upload into WordPress.
This would create a 2x2 ad block that you could use in the top sidebar.