Page not taking partial post back on server

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ASP.NET

Hi I am suffering from a very unusual problem. I created a functionality where a dialogue box appears on client's screen so that he can download his respective data.
Because of content.disposition I was unable to hide progress image so I used img alt feature where I wrote "Processing your request...". So now when user hits the button than initially for some time progress image appears and after that when data is mined from the data table than my page automatically takes a kind of post back by which image gets hide and its alternate text appears. All this was pretty ok as far as user was notified that something is happening at the back for his data but when i uploaded the functionality to live than image doesn't gets hide at all and even if the dialogue box appears on the screen than still the image stays there on the page. I am not getting why this is hapening, I am not even using any update panel.

Any help will be sincerely appreciated.