I'm, not sure what code change limitations you have but you can accomplish this without a rollover image by using CSS.
The CSS would be something like this:
Code:.fade{filter:alpha(opacity=50); -moz-opacity:0.5;}
.normal{filter:alpha(opacity=100); -moz-opacity:1;}
Then you define the image class like this:
Code:class="fade" onmouseover="this.className='normal'" onmouseout="this.className='fade'"