when we trigger the event mouseover image should brighten

You can do this by  using JavaScript. Add the following code in your html page to achieve the reult.   

function makevisible(cur,which){
strength=(which==0)? 1 : 0.6
if (cur.style.MozOpacity)
else if (cur.filters)

<p><img style="filter:alpha(opacity=60);-moz-opacity:0.6" onMouseover="makevisible(this,0)" onMouseout="makevisible(this,1)" src="images/thisImage.jpg" border="0" target="_top" alt=" " onmouseover="window.status=' '; return true;" /><br />

Added on September 16, 2008 Comment