It is difficult for me to see where exactly the problem lies, but if you were going to follow Joleen's suggestion of adding pixel widths instead of percentage widths, you may also try something like this:
<table width=100%>
<tr>
<td align=center>
<table width=800>
<tr>
<td align=left>
Your content here. blah blah blah. your image
sizes must be less than 800 pixels wide, etc. etc.
</td>
</tr>
</table>
</td>
</tr>
</table>
The outer table will make sure that you utilize the whole viewing area of the browser. While the inner table will contain your site contents.
Anything in the outer table can be aligned the way you like it. I prefer doing it this way differently than adding the center attribute in the body, because sometimes you want to center ONLY that specific table cell and not the whole page.
As dl65 requested, a URL to the page would also help, so we can check if you have a specific problem somewhere.
Regards,
FolkLore