Reset The Width Of Image

I am trying to reset the width of the image while loading:

<img src='s.jpg' onload="javascript:if(this.width>120)
this.width=100;">

It's ok. And then I am going to make the code more flexible

<img src='s.jpg'
onload="javascript:if(this.width>javascript:window.document.body.offsetWidth)
this.width=50;">

this time, I compare the image width with the offsetWidth, however, I
found the code above didn't work no matter how small offsetWidh is
made.

BTW, I would like to make the width of image fit the current width of
the document, only HTML works fine under IE

<img src='s.jpg' width="100%" LEFTMARGIN=10>

But when applying the above code to the image of small size, it always
try to strech the image ,which will then look very bad.

I am trying to apply the code on condiiton: when the width of image is
smaller than a given value, do nothing; otherwise add "width=100%
LEFTMARGIN=10". Here is the javascript I tried

<img src='s.jpg' onload="javascript:if(this.width>100)
javascript:document.write('WIDTH=100% LEFTMARGIN=10');">

However, it didn't give the result I want.