Adding to Jens Peters reply, an additional thing you can do is to use a background colour that's the same as the 'background image background colour', if that makes sense.
You would use a style for the body something like:
body {
background-color:#6699FF;
background-image:url('images/mybackground.jpg');
background-repeat:no-repeat;
background-position:left;
}
Here my background image has a background colour of #6699ff.
The background-position can help and if it's a small image, 'center' might work.
HTH
Ian
http://www.ew-resource.co.ukhttp://www.fp-resource.co.uk
Ian Haynes