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