Yes I missed to check the submit data connection, now I checked the overwrite checkbox it is working perfectly. Thank you very much for your kindly help.
Add on to this question. I have one more question
How can I add overwrite parameter in below formula?
concat("SaveToSharePoint /url=http://server/site/docLibrary /xpath=/my:myFields/my:Images/my:Image /name=", Result)
Thanks In Advance
Praveen Manne