There's several ways of going about that actually, including adding a jquery scrollover type of animated effect. The simplest code to do so though is:
Code:
<img src="image.gif" width="" height="" border="" onmouseout="this.src='image.gif';" onmouseover="this.src='image2.gif';" alt="" />
Where "image.gif" is the image that loads on first appearance, "image2.gif" is the image shown on mouseover. You can define width/height/border/alt
There's better ways to do it of course - but you asked for the easiest way
HTH
Jacquii.
--- depending on your style - you will find the logo code in your header template.