How do you add a fadeIn - fadeOut transition to a swap image

[Hawke001]

I found out all sorts of ways of adding a nice fade transition to a rollover image, but none actually say how to apply the script to an html documentthis is what I've got so far.. how can I add a fadeIn, fadeOut script to that? instead of just flicking from image to image\[code\]<img src="http://stackoverflow.com/questions/15718994/images/portfolio.jpg" alt="" width="155" height="150" id="portfolio" onmouseover="MM_swapImage('portfolio','','images alt/index-alt_12.jpg',1)" onmouseout="MM_swapImgRestore()" />\[/code\]