You have to wrap CSS in a <style>
tag, like this:
<style type="text/css">
#cf6_image {
margin:0 auto;
width:450px;
height:281px;
transition: background-image 1s ease-in-out;
background-image:url("/images/Windows%20Logo.jpg");
}
#cf6_image:hover {
background-image:url("/images/Turtle.jpg");
}
</style>