When I am trying to change my image in the following way
System Settings > Account details > click to change your image

It shows "Your administrator has disallowed changing your image"

but I am the administrator,its aprivate laptop,only I am using it ,there is no other user.
Thank you

You need to open "Login Manager" settings ( located in System Settings ), open the "Users" tab, and change "User Image Source" to "User, system"