Hi,
The following code should correspond to what you have described: a 400x100 image followed by a table having width of 100%:
<html>
<body>
<img src="" width="400" height="100">
<table width="100%" style="border: 1px solid black">
<tr>
<td>table</td>
</tr>
</table>
</body>
</html>
The resulting PDF looks ok, you can test it at
http://pdfcrowd.com/#convert_by_inputPlease, could you post your html code that does not work so we can look into it? Thanks.