I have now got this advertisement code worked out to be used in MG Autumn style for phpBB3.
You need to edit the Google details to your own of course and as well as that you can change the details to run banner ads if you want to as it is not necessarily made just for Google ads.
Open viewtopic_body.html
Find:
Before add:
<!-- IF postrow.S_FIRST_ROW == 2 -->
<tr class="row2">
<td align="center" valign="top">
<b class="postauthor" style="color:#1845ad;">G<span style="color:#ad1400;">o</span><span style="color:#c69e00;">o</span>g<span style="color:#31b639;">l</span><span style="color:#ad1400;">e</span></b><br /><br />
<img src="{T_THEME_PATH}/images/google.jpg" alt="google adsense" />
</td>
<td width="100%" height="25">
<table width="100%" cellspacing="0">
<tr>
<td class="gensmall" width="100%"> </td>
</tr>
</td>
</tr>
<tr>
<td valign="top">
<table width="100%" cellspacing="5">
<tr>
<td>
<div class="postbody">
<script type="text/javascript">
<!--
google_ad_client = "pub-9190636154929816";
google_ad_width = 728;
google_ad_height = 90;
google_ad_format = "728x90_as";
google_ad_type = "text";
//Googleads, 728x90
google_ad_channel = "0814071977";
google_color_border = "8FBC8F";
google_color_bg = "E8E8E8";
google_color_link = "E7E7E7";
google_color_text = "000000";
google_color_url = "0080000";
//-->
</script>
<script type="text/javascript"
src="h**p://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>
</div>
<table width="100%" cellspacing="0">
<tr valign="middle">
<td class="gensmall" align="{S_CONTENT_FLOW_END}">
<a href="#">{INFO_IMG}</a>
</td>
</tr>
</table>
</td>
</tr>
</table>
</td>
</tr>
</table>
<tr><td class="spacer" colspan="2"><img src="{T_THEME_PATH}/images/spacer.gif" width="1" height="4" alt="" /></td></tr>
<!-- ENDIF -->
Save this image and upload it to Styles > Template > Theme > Images