I have an image of a twitter icon in black and white and a second image of it in color. When the mouse hovers over it changes to the colored one. However I can only choose between 'fast' and 'slow' speeds. I tried putting in numbers but it didn't work. Is there a way to make the change even faster.

Code:
<script type='text/javascript' src='../jquery.js'></script>
<script type='text/javascript'>
$(document).ready(function(){
 
$("img.a").hover(
function() {
$(this).stop().animate({"opacity": "0"}, "fast");
},
function() {
$(this).stop().animate({"opacity": "1"}, "fast");
});
 
});
</script>
<style>
div.fadehover {
	position: relative;
	}
 
img.a {
	position: absolute;
	left: 0;
	top: 0;
        z-index: 10;
	}
 
img.b {
	position: absolute;
	left: 0;
	top: 0;
	}
</style>
Code:
        <div class="fadehover">
	<img src="../../../twitter1.png" alt="" class="a" />
	<img src="../../../twitter2.png" alt="" class="b" />
	</div>