This can be done in two methods,
Create two images with the logos,
Set a background for a H1, or something else to display
My example is this
<style type="text/css">
#Header{
background:url(urlof the image);
width:somewith;
height:someheight;
overflow:hidden;
text-indent:-90000px;
}
#Header:hover{
background:url(url of the second image);
}
</style>
....
<h1 id="Header">This is the header</h1>
....
This is the second method. Use a large image with both logos. one below the other.
Use the following code to make it.
<style type="text/css">
#Header{
background:url(urlof the image) top;
width:somewith;
height:someheight;
overflow:hidden;
text-indent:-90000px;
}
#Header:hover{
background:url(urlof the image) bottom;
}
</style>
....
<h1 id="Header">This is the header</h1>
....
I would really suggest you to use the second method :)