To do this, watermark your photos using another app BEFORE you upload them to your social networks and website. Here are a few options of apps that you can use:
Our favorite, due to the easy to use interface (and the availability of a free lite version).
A bit clumbsy, but it’s a free way to watermark your photos easily before adding them online.
Able to upload your own image and add transparency.
Only able to do use font watermarks, but still gives some control on font and font effects.
Texts on Photo
A paid application, but pretty easy to use and has awesome font effects that allow you to be creative.
The trick is setting up a watermark in your chosen app and then quickly adding it to all of your important photos before it goes out into the big world of social networking.