The issue that you're running into isn't filemaker, it's the web code itself.
The HTML and body tags get some default styling applied to them by the web browser. If you take your code and write it out in just a regular html file you can see the padding as well.
Try adding this to your style tag:
html,body{
padding:0px;
margin:0px;
height:100%;
width:100%;
}
This will tell the web page that your html and body tag need to take up the entire browser window (the height and width rules) and that they should have no padding or margins. This will make sure your image tag is flush against the boundaries of the web viewer.
Hope this helps!