These animations are typically done with Javascript. Rather than writing the Javascript by hand, it is probably easier to use the jQuery library, which includes the ".animate()" method. The animate method requires you give the destination css properties as parameters, like so:
(Since you wrote you are not familiar with Javascript, I've included everything you need to include the jQuery library)
<script src="http://www.google.com/jsapi" type="text/javascript"></script>
<script type="text/javascript">google.load("jquery", "1.6.4");</script>
<script type="text/javascript">
$(document).ready(function(){
$("#image-div img").live({mouseenter:myfin,
mouseleave:myfout});
});
function myfin () {
$(this).animate({height: 200, width: 200},1000); //1000 = 1 second animation
}
function myfout () {
$(this).animate({height: '', width: ''},1000); //1000 = 1 second animation
//setting the height and width to '' will clear the inserted style, leaving you with your original style
}
</script>
Then all you should need is to set the height and width to 100px in your CSS, and remove the #image-div:hover definition.
If you would like to animate using class definitions in your CSS file, you can do so using a jQuery plug-in. See the following question for help on that:
jQuery.animate() with css class only, without explicit styles