ok, you need to id your <img> so we can find it. You'll need this in your html:

<img id='nottoobig' ...>

Now, a little code. I'm making the following assumtions in this code, first that you want to check the height and the width, second that it won't scale an image up if it's too small.

function checkImageSize() {
var img = document.getElementById( "nottoobig");
var bdy = document.body;
var a = img.width / img.height;

if( img.width > bdy.clientWidth *0.9) {
img.width = Math.floor( body.clientWidth *0.9);
img.height = img.width *a;
}

if( img.height > bdy.clientHeight *0.9) {
img.height = Math.floor( body.clientWidth *0.9);
img.width = img.height *a;
}
}

It's untested but should work. It'll do a double pass - it'll set the width, and then still check the height afterwards, that should work with the example below.

You'll also have to add an onLoad handler to call it.