Postpone server event

I have 2 designed button images, where I want the second to be shown immediately after the button is clicked, for half a second or so before the OnClick event of my ImageButton control is executed. But I don't get it work.
If I call a javascript function that changes imageurl of my imagebutton in the OnClientClick event and then in the OnClick server function use System.Threading.Thread.Sleep(500) then the OnClick event is postponed but I never see my second image. The only way I manage to see my second image is to put 'return false' last in my javascript function.




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