Need Dynamic Watermark on Alfresco

Hi...I am a new come using Alfresco Community Share 5.2 for about 3months now. For now I want to add a watermark in every document uploaded with different format for each site and folder. the watermark will be place in document that download or preview by some one. I need to know also how the implementation since I am new with this CMS and don't know how it work. So please give me some thought or any idea related to that. Thanks&Regards, Sun