[monolithed]

Что-то типо такого:

<script type="text/javascript">
window.onload = function(){
    var img = document.getElementById('div').getElementsByTagName('img'), i = img.length;
    while(i--){
        var computed = window.getComputedStyle ? getComputedStyle(img[i], null) : img[i].currentStyle;
        img[i].style.width = parseInt(computed.width) >= 100 ? '100px' :'';
    }
};
</script>

<div id="div">
    <img src="http://javascript.ru/forum/images/ca_serenity/misc/logo.gif" alt="" />
    <img src="http://javascript.ru/forum/images/ca_serenity/misc/logo.gif" alt="" />
</div>

или такого:

<script type="text/javascript">
window.onload = function(){
    var img = document.getElementById('div').getElementsByTagName('img'), i = img.length;
    while(i--){
        img[i].style.width = img[i].offsetWidth >= 100 ? '100px' : '';
    }
};
</script>

<div id="div">
    <img src="http://javascript.ru/forum/images/ca_serenity/misc/logo.gif" alt="" />
    <img src="http://javascript.ru/forum/images/ca_serenity/misc/logo.gif" alt="" />
</div>