fading background using CSS

hi friends ,


i have two images

1) back.jpg
2) bar.gif



I want that when i press submit button then back.jpg comes to screen with 50% opacity and bar.gif opens in the center of that images to show that processing is occuring

Can anyone help me out

But when you submit, you will also change the session, won't you? so that you can not use javascript.

When i pres submit button , the processing was taking 8-9 seconds
so just wanted to show some processing screen but

i found on internet using CSS opacity function and solved it