Hi Suresh,
You can change the background image by using UI theme designer by accessing following URL.
http:/host:port/sap/bc/theming/theme-designer?sap-client=clientID
There you can create a custom theme and there you can customise launchpad as per the reuirement. But if you want to customise backround image,logo etc .. in Fiori launchpad login screen, please refer to below SCN Post
http://scn.sap.com/docs/DOC-59371
Regards,
Arun.