Display LightBox Loading image on Page when AJAX UpdatePanel is loading or doing PostBack in ASP.Net

Sir,

 I have update panel on the page to avoid postback and placed a dropdownlist control within that update panel. DrodownList is filled with Category names and also it's autopostback set to True.

Now whenever dropdownlist index change's on select, untill page loads complete  data it should show mesage as "Loading.... in center and middle of the page and background should become bit transparent". And when page load completely with data then that background and message should get disappear..

Is it feasible...any suggestions would be helpfull.