img onmouseover filter or change image

1.Filter
<img src="a.jpg" onmouseover="this.style.filter='alpha(opacity=\'50\')';" onmouseout="this.style.filter='alpha(opacity=\'100\')';">

2.Change Image
function changeimg(id,imgf){
 var chid = document.getElementById(id);
 chid.src = imgf;
}
<img src="a.jpg" id="imgab" onmouseover="changeimg('imgab','b.jpg');" onmouseout="changeimg('imgab','a.jpg');">