Hello, Sue Mak.
Probably you need to refresh (invalidate) a server cache in tx. SMICM.
My own advise would be to follow the steps which Bruno Garcia suggested in very first answer. Because this is a standard way. And replacing standard images like SAPNetWeaverLoginTitle_wo_Subtitle.jpg or NetWeaver_Default_Picture.jpg is not a perfect solution. For example, are you sure that SAP doesn't use it anywhere else? Obviously, it doesn't. But anyway...