Unable to find Image Control in JavaScript when working with MasterPage in ASP.Net

Theres is requirement in my project to upload file and display immedia to image control without refresh on postback.        i go through your code or sample demo which you posted........

actually its working but when  there is child page of master page  it giving me error like.. "imgDisplay is null" plz help me to resolve this......

error : Unhandled Exception: imgDisplay is null

thanks

walmik chavan