I have restricted the access for users to view the photo in real size. We have an option to set permission for all groups in CP.
In that i have restricted the access for users, guest, friends, checked to view photo in real size. So that even if they click the link it will ask them to contact administrator.
So i think it'll be fine for me..... Anyways thanks for your suggestion.... I'll try the watermark method if it suits me i'll go for it.
"I don't like to commit myself about heaven and hell - you see, I have friends in both places"